FOX-P2;1945559; said:First get an accurate meter and forget about the chemical tests.
Normally with pH below 5.5 you would get a false ammonia reading ( ammonium) . Your filter bacteria become biostatic at that low pH and do nothing, but with a zero reading your problem is probably low buffers. Do some water changes without the R/O water.
Also as mentioned, carbon needs to be replaced frequently as it reaches a saturation point. I think no carbon is better, instead make room for more bio filtration
does that mean that my tank has no biological filtration at present?