Hydroponic Resovoir Nutrient Solution Dosage and Top UP ( I use bacilus subtlis to prevent root rot , hydroguard is to expensive for me and not easily avai

It is hydro, so everything should be able to enter the plant. The nutrient uptake part (2nd) is pretty interesting read -- https://www.agry.purdue.edu/ext/pubs/AGRY-95-08.pdf light and heat are your rez's enemy. either go with sanitization (h2o2, for example) or beneficial microbes to keep it under control. For heat, it's best it's not in your tent, preferably piped in from a cooler area. How well it is maintained will determine how lazy you can be about emptying and refilling reservoir, though i wouldn't go longer than 10-14 days - topping off along way. try to avoid any unusual smells (depending on stuff you add = "normal smell") .. that'll help you determine how long between. your local variables will dictate microbial growth. i'd want to top off with same ratio and concentration as it was originally as opposed to water-only added. can always make sure EC hasn't spiked.. if similar you can assume similar ratios since you don't keep the rez water for long periods of time. If you see it spiking, can go with water-only. If your ratios match rate of use in the plant, should not have any shifts. maintaining consistency is all you need at that point. tweak until you get there... I keep a 55-gal rez but not for dwc. From my experience, water fouls in a rez much faster at low levels. so, between complete refills, maintain full water level even if topping off is a small amount daily or every other day. whether it;s surface to volume ratio or whatever, seems to hold true for me. I never let my rez drop below 50% for longer than 15mins -- 'last' irrigation before refull drops to about 25% then i add 75% of full volume back. If i get lazy and let the 25% level sit over night i have to clean it sooner.