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How Often Should You Change the Water in a Hydroponic System?

Added 27 June 2022

Hydroponic cannabis systems inherently require more involvement from the grower, considering the grower is in control of all of its aspects, including the nutrient cycle, light, humidity, et al. 

Most of the intensive things are taken care of by some growers, but many new growers often fail to grasp the basics. One of them is the frequency of changing the water or nutrient solution in their hydroponic setup.

In this article, learn about when and how to change the water in your hydroponic system. In addition, you'll learn a few general tips to take care of your setup. 

Topping Off vs Full Change: When to do a Full Change? 

nutrient reservoir

Many growers wonder why they can’t just top the solution so the reservoir stays full. After all, it makes things infinitely easier, right?


Unfortunately, although topping frequently helps, your plants can face serious issues if you don't change the water at all, including:

  • The nutrient solution can become diluted by topping, which can cause malnutrition or nutrient deficiencies in your plant
  • Topping over many days can also cause an imbalance in your nutrient solution because some nutrients get absorbed more quickly than others 
  • The same can also cause fluctuations in the pH, which can hamper the roots’ nutrient absorption 
  • Lastly, old water can fester fungi and bacteria, which can translate into diseases for your plants 

All in all, simply topping off the reservoir does not cut it, and in severe cases, it can stunt the plant’s growth or even kill it. 

That does not mean you don’t top off your reservoir tank at all — instead, you can top it off every other day to keep the volume consistent; however, it's a temporary measure. 

Once you have added the same volume of water as your reservoir’s capacity, it is time to completely change the nutrient solution. For example, if your reservoir holds 10 liters of solution, and you add one liter of water every day, you will have to completely change the water after 10 days.

Do this and you will ensure your nutrient solution has the ideal levels of nutrients, pH, EC, etc. for healthy growth without the risk of bacteria or fungi.

Factors that Affect the Frequency of Full Change

hydroponic systems

Apart from a topping the volume of the reservoir, you should also understand some other factors that may require a full change earlier, such as the following:

  • If the container is too small, the water will deplete quicker than in a bigger container
  • Uncovered containers also lose more water compared to covered ones due to evaporation
  • Some strains are also more nutrient-hungry, so your cannabis strain may consume more nutrients than expected 
  • Plants also consume different quantities of nutrients depending on their growth stage
  • A warmer grow room can also lead to excess evaporation

It’s best to keep an eye on the nutrient solution. Over a few weeks, you will understand your plant’s nutrient needs and consumption, and doing a full change will become much easier.

What are Some Other Instances that Require a Full Change?

hydroponic cannabis

Apart from the basic full change, as mentioned above, there may be some other instances too that require you to completely change your nutrient solution. This is called the abnormal full change, and here are some instances when you may have to do it. 

When pH is Out of Balance

pH is the concentration of hydrogen ions within a solution, which dictates its acidity or alkalinity. pH is a complicated chemistry subject, but when it comes to hydroponics, it is pretty straightforward — you just need to maintain the right pH balance in your nutrient solution.

Cannabis likes a slightly acidic nutrient solution, so the best pH range for cannabis is between 5.5 to 6.5. Within this range, the roots can absorb the nutrients easily, but if the solution is out of whack, it can inhibit nutrient absorption. So, the right pH is essential to maintain your plant’s health and growth.

Since pH is directly linked to your plant’s health and growth, you need to test the solution’s pH every day, whether you are topping it off or not. You can use a simple pH pen to test the pH. pH test strips are also a good option but they are cumbersome and not so accurate to read. 

If you notice the solution’s pH balance is off, you need to quickly rebalance it. You can do so with pH Up or pH Down solutions, which are readily available in most gardening stores. Simply add them to the solution as instructed to bring the pH back to the right levels.

pH rebalancing solutions are only a one-time aid, but they don’t solve the fundamental issue. Plus, using a lot of pH rebalancers is not a good idea. So, if you notice that the solution’s pH is too far off, or keeps changing even after rebalancing, you need to do a full change of the water. 

When doing a full swap, ensure you are using the right quantity of nutrients and neutral water in the following solution. Always test your water source’s pH too, since some sources tend to be more alkaline.

When EC is Changing in the Solution

EC refers to electrical conductivity, i.e., how well an electrical current can travel through your nutrient solution. The EC test is used to test the number of nutrients present in the solution, since the higher the concentration, the higher the electrical conductivity. 

It is a good idea to test your nutrient solution’s EC daily, along with the pH. 

Your solution’s EC may sometimes change, particularly since topping off dilutes the solution or some nutrients get left behind. But you need to constantly check the EC and ensure it stays consistent throughout the growing cycle.

If the EC gets too high, it can raise the osmotic pressure in the root zone, or it can make the ions toxic to the roots, affecting the root’s nutrient uptake. On the other hand, low EC can cause nutrient deficiencies in the plant. 

The ideal EC ranges for the plant are:

  • Seedling stage: 0.8 to 1.3
  • Cloning stage: 0.5 to 1.3
  • Vegetative stage: 1.3 to 1.7
  • Flowering stage: 1.2 to 2

Again, if your EC levels are off by a margin, you can always add more nutrients to bring it up, or dilute the solution with some water to bring it down. But if the EC is too off, diluting or concentrating it won’t help — you need to do a full change of the nutrient solution.

Whenever You Change the Nutrient Ratios

You must be following a specific nutrient ratio for your plant, and during its growth, you may have to change it. If that is the case, you need to completely flush out the old nutrient and prepare a new nutrient mix from scratch.

This also applies if you are adding any new fertilizer from a different brand. 

You Notice Signs of Nutrient Imbalances or Bacteria

Another reason why you would need to do a full change of your nutrient solution is when you notice any signs of nutrient deficiencies, lockout, or bacterial growth on the roots or the tank.

An obvious sign of nutrient imbalances is chlorosis (yellowing of the leaves), purple tint developing on the plant, leaf or root necrosis, or stunted growth. You need to find the exact nutrient (or lack of it) that is causing it, followed by doing a full nutrient swap. 

It’s a good idea to learn as much as you can about signs of nutrient imbalances or bacterial infestation while growing cannabis. 

How to Do a Full Change of the Nutrient Solution?

lifting roots from reservoir

When doing a full change of the nutrient solution in your reservoir tank, you need to follow some steps, such as the following:

  1. Remove the growing tray if it is located above the reservoir
  2. Drain all the nutrient solution from the reservoir
  3. Clean with reservoir tank thoroughly to wash out any residues
  4. Rinse the same with water
  5. Refill the reservoir with a fresh nutrient solution with the right pH and EC ratings
  6. Reassemble any pieces of the system you may have removed

Here, you may not want to throw away the old nutrient solution — a lot of work has gone into it. Also, tossing it into the drain is not good for the local ecosystem. Instead, you can mix it with tap water in a 50:50 ratio and spread it over your garden flowerbed. The plants will benefit from the residual nutrients. 

Tips to Maintain your Hydroponic Reservoir

drip system

1. Keep an eye on the levels

Make sure that the levels of the nutrient solution are always full. Place a lid on top of the reservoir to reduce evaporation as much as possible and also to prevent light from entering the reservoir. Light in the reservoir can encourage algae that is not good for your plants.

Using large amounts of nutrient solutions will ensure that the plants rarely run out of nutrients. In addition, large volumes will act as buffers if you mess up and add too many nutrients to the solution. However, note that you have to maintain the proper nutrient ratio. Remember, we are talking about the volume and not the concentration. 

To prevent nutrient deficiency you can top up the reservoir as and when you see the levels diminishing. Also if you want to change the nutrient solution, make sure you use all the previous solution so that there is no wastage. Commercial nutrients are expensive so you will save money by following this practice regularly.

2. Get the right reservoir

Firstly, you should have a reservoir large enough to feed the cannabis plants. Although the nutrient requirements vary from one strain to another, most cannabis plants are hungry and feed a lot especially when they enter the flowering stage. Thus, you should have a reservoir that is slightly larger than the nutrient solution it can hold.

But how do you calculate the reservoir capacity? Well, it depends on a lot of factors including the number of plants you're attempting to grow. The size of your reservoir can help you grow stunning plants or make it a major problem where you lose all your plants.

For example, if your reservoir size is too small, your plants will not be able to get enough nutrients and water. The nutrients serve as food for the plants. Just like you need food to survive and grow, the plants will need a reservoir that gives them enough nutrients to survive.

To calculate the capacity of the reservoir the first thing is to decide the number of plants you want to cultivate. You will also have to keep the strain size in mind. For example, some plants don't grow more than 3 feet in height whereas others may grow more than 4-5 feet and become gigantic plants.

Here's what you can consider approximately when you set up the reservoir:

Small plants - At least 2 liters of solution per plant

Medium plants - 3-5 liters of solution per plant

Large plants - At least 10 liters of solution per plant.

Note that your plant may consume more, so you need to start with these amounts.

In addition rather than measuring the nutrient solution every time you fill the reservoir, you can mark the levels using permanent markers to make it easier.

You should also consider other factors such as the temperature and humidity that play a role in choosing the size of the reservoir.

For instance, humidity impacts the way the plant absorbs water in hydroponic systems. Plants breathe out faster when the humidity levels are low, which means that the roots drink more water to replace the water that was lost. On the other hand when the humidity levels are high plants don't breathe out slower, which means that the roots don't absorb too much water in such conditions.

Apart from humidity, temperature also plays a key role in hydroponic setups. Plants rely more on the water when the temperature rises. In addition, you should also consider maintaining the root zone temperatures as it puts enormous stress on the plant if there's an imbalance.

Last but not least, you should also consider the amount of light the plants will receive. For example, if you have extremely bright lights the plants will use more water during the process of photosynthesis. The brighter the light, the more water the plant uses.

3. Reservoir nutrient solution temperature

You should always keep an eye on the temperature of the reservoir. Although there's a margin of error and you can experience slight fluctuations, it must range between 60°F to 75°F or 15°C to 24°C. 

If you don't maintain the temperatures properly, the plants can experience sudden shock or they will not be able to absorb the nutrients easily.

Also as a hydroponic grower, you understand the importance of oxygen in the reservoir. So you can maintain the temperature in such a way that the plants get more oxygen. For example, water holds more oxygen when it is at 15°C or 59°F. The levels of oxygen reduce slowly as the temperature increases. 

In addition, the reservoir maintained at such temperatures will keep the humidity levels in check in the grow room and also control transpiration. What's more, it also helps the plant to take in the nutrients more easily. As you may already know, installing an air pump will also help introduce more oxygen into the reservoir.

If the temperature of the reservoir exceeds 30°C or 86°F, it means it holds very little oxygen, which can be detrimental to your plants.

Also, if you live in a cold climate it's best to use warm water in the reservoir rather than using an air heater as it will heat the room slightly. However, do not use hot water as the roots will be cooked. Again, remember, the temperatures shouldn't go down below 15°C or above 24°C.

4. Irrigation

There is no set rule for irrigating your plants while growing them hydroponically because it will depend on various factors including the size of your plants, growing medium, climate conditions, etc. Once you grow several plants you will be able to adjust the irrigation cycle based on how your plants respond to it.

Hydroponic growers should pay special attention to the growing medium they use. For instance, if you use growing mediums that drain very easily, you will need to water more compared to other mediums. Depending on your setup, you can fix an irrigation cycle that goes on for a few minutes a day and then extend or reduce the time based on how your plants grow.

5. Nutrient irregularities

Many things can go wrong when you are growing cannabis plants hydroponically in a reservoir. For instance, the plants can absorb excess nutrients and experience nutrient burn or a lockout, where they won't be able to absorb nutrients anymore. On the other hand, if the solution doesn't contain a proper balance, the plants can also experience nutrient deficiencies.

Therefore it is extremely important to maintain your hydroponic setup frequently and understand how the plants grow so you can prevent most of the problems in the first place.

It is not uncommon to experience nutrient deficiency and nutrient burn occasionally but if your plants are always struggling with some of these issues then it's time to take a good look at your setup to figure out where you could be going wrong.

Firstly, check the tubing and irrigation fittings to ensure everything is working fine. Check for leaks in the system or any sort of blockages that could be disrupting the flow of nutrients. A few obvious fixes should help the plants absorb nutrients at a proper rate and prevent deficiencies as well as nutrient burn.

If you notice some plants experiencing nutrient deficiencies consistently, ensure that they are fed properly. In addition, make sure that the drainage channels are not parked in stagnant solutions as this will lead to root rot pretty quickly.

Finally, check the pH and make sure it is in a range of 5.5 to 6.5. If the pH is incorrect, then flush out the water and start with pH-balanced nutrients again. However, if the pH is in the correct range and you still experience nutrient problems, then it may be time to change the solution completely and start again, just like you would when there is a pH problem. 

If the problem persists even after you have done everything, then you can try a different fertilizer brand. Talk to other growers that are also growing hydroponic cannabis and figure out which nutrient works the best. Note that while some nutrient brands may be perfect for some growers, it might just not work out for you so changing the brand and starting all over again may be a better option.

Compared to growing cannabis in the soil, you must take nutrient deficiencies or nutrient burn very seriously when growing in hydroponic setups because there is no buffer to save the plants. 

Plants growing in soil have enough time to recover but the same is not true for hydroponic plants. A minor deficiency can spread very quickly or a nutrient burn can kill the plants in a very short period. 

Also, sometimes it can be very difficult to exactly guess what the plant is suffering from especially if there are two or more things that have gone wrong simultaneously in the system. Therefore you will have to start fresh if you are confused about what could have gone wrong.

Fortunately, it is easy to take care of most of these problems and you can start by flushing your system at least twice with a fresh nutrient solution. Doing so must help with most of your problems.

Summary: How Often Should You Change the Water in a Hydroponic System?

Growing cannabis in a hydroponic system is a terrific method, but it is as much of a science as it is an art. And precision can take you a long way with it. So, you must be on your toes when it comes to changing the nutrient solution regularly. 

Usually, the nutrient solution can last around ten days, but learn how your hydroponic system behaves and take measures to keep it topped off for a while and change it regularly. 

And if the pH and EC levels are too far off, or you notice any signs of nutrient imbalances or bacterial growth, it is a good idea to fully change the nutrient solution even if you replaced it just a day ago.