also I might have to leave these plants. so I migh...

it can if stagnant quite quickly... keeping it cold, dark and stirring will slow that process down (microbial gowth). if you are talking a week, probablay gonna be okay, but anything much longer you might want to consider more than just a big barrel of fertilized water. something with a lid to reduce evaporation will avoid one cause for concentration to change - the other being microbes eating your nutes and wreaking havoc on pH. got a power head for an aquarium or some air pumps and tubing to make bubbles? otherwise make they stir it each day... that will oxygenate it at the least. probably have some film on it by end of week... doesn't necessarily mean doom but leads to potential problems if the wrong stuff starts gowing in your medium.