The problem with hobby-level phosphate tests is they don't read down to NSW
levels of 0.01-0.02, thus your kits give you a reading of 0 simply because that's as far down as they can go, so it ain't necessarily true that you have no phosphate in your tank. Even .05 phosphate can help cause a nuisance algae outbreak.
Only reagent-based test kit I know of that reads down to NSW
phosphate levels is made by Merck. I did a bit of searching for it before I posted, so I don't know if its even made anymore.
LaMotte makes a low-range phosphate reagent that gets close, down to .03, but both the reagent and the colometer to read the results are far from cheap. You wouldn't get much change from $1000 US for the pair.
You should buy a backflush kit for your RO
unit. They are inexpensive, around 20 dollars. The nitrate coming out is the result of nitrafying bacteria within your unit that have taken up residence on your membrane. You obviously have chloramine in your tap water. The RO
takes out the chlorine, and then there's ammonia. You're seeing the bacteria at work when you see that nitrate.
You should backflush every month or six weeks, depending on how hard your tap water is. Very hard do it sooner, very soft do it later.
Backflushing will solve your 25 ppm
nitrate problem, and extend the life of your membrane markedly, as in several years before needing replacement.
Did you wash out the inside of your trash cans with soap and water? Reason I ask if you're having contamination in it, it's most likely the mold release agents precipitating into the water. All those cans are molded, and there are agents which allow the molded can to pop out of the mold. Those agents don't matter if your use the cans for trash, they do if you put tank water in them.