This question is a testament to how confusing things can get when you don't have the experience. I normally keep my reservoir topped off with water and add nutrients only as needed. I don't have an EC meter, so I talk to my plants; looking for obvious problem signs. The problem is that my babies are in the third week of flower and it's becoming too difficult to keep up!!!! Many of her fan leaves are yellow and necrotic (Shriveled up and dieing) It looks like Nitrogen/Phosphorus deficiency. So today, I finally decided I need to top with "quarter" strength nutrient solution when a simple, but problematic issue of confusion occurred... This might sound stupid, but WHAT IS 25% Solution? Let me give you an example; I have a 15 litre reservoir that calls for 27ml of grow, micro & bloom (1:1:1), I divide 27ml by 4 and get 6.75ml. 25% solution right? WAIT! I am making only a gallon (3.78 liters) of 25% solution, not 15 liters... So, I need to adjust this to fit my quantity, and when I do, I get vastly a different answer. I did this by using a proportion (a mathematical way of scaling ratios correctly) ml 27 . n L 15 3.78 (I solve for 'n' using cross multiplication [keeping ml on top and the res size on bottom in both fractions, supplementing 'n' for the unknown ml value]) Simplified: 27 x 3.78 = 102 15 x n = 15n 102 divided by 15n = 6.8ml NOW 6.8ml sounds pretty close to what I got before, however; this is still 100% nutrient strength. Remember that I only scaled down the ml to fit my gallon, if I were to use 4 of these, the total nutrients would equal 27ml... This simply means that the EC would remain the same in this ONE gallon as my 100% 4 gallon reservoir. Now, I need to divide 6.8ml by 4 = 1.7ml. So here we are... Should I top with solution A (6.75ml / gallon) or solution B (1.7ml / gallon)? The answer is Solution B. The first method only scaled down my reservoir size to 1 gallon and DID NOT reduce the total percent of the solution. Plants use 1% nutrients to 99% water, so it is important to understand how to feed your plants correctly. Seeing as how there is no guide on how to do this, I have made one. Maybe it will help someone or maybe, it just too damn difficult to follow for your humble hydroponic pot farmer lol I hope that people aren't looking at this thinking... 'What the hell is a proportion any-damn-way? This guy is freaking crazy!' It's simple, just take your time reading my post and you'll be cool
May i suggest the Lucas system,it uses GH 3 part or other 2-3 part nutes.(but no grow part either) This system has a very simple add back calculation too. A simple cheap ppm meter will keep solutions at the optimum level daily.(BEST option IMO) Using 1/4 nutes for 1/4th the solution makes sense and using 1/4th of that is 25% strength,very simple there. There is formulas for calculating just how much nutes to add back with water w/o using a meter but i have never used it in lieu of the more accurate meter method.
You are making it waaay too difficult! I use empty gallon jugs for my water. I fill them up and let them set uncovered for a day or two to let chlorine, etc. evaporate out. I mix my nutes at whatever strength I need for the growth stage, and top off the rez. Don't worry about using a 25% mix or whatever. I think you will find the gallon jugs will make it much more simple....
I have never grown hydroponicly, but i have heard of ploblems with nutrient burn/buildup when topping off resivour between changes with nutrient enriched water. I be told to just add Phed water to the mix untill the next drain and change. w0rldreign if you are unsure I would just drain the resivour and replace with proper strenght nutes.