wow, fish gasping for air and you have cero ammonia and nitrite. humm. What you use or the store use to test the parameters? You can dose ferts, still, low light and no CO2, there maybe not need to dose macros, maybe veryyy low quantities not are bad. Macros and micros are the food for the plants, if the plants not eats them fast enought mainly the macros will go up overtime. Do you make w/c?
I have a 55G el natural (NO CO2, Sun light, still good canister), with lots of plants (swords, valls, egerias, anubias), few fish, substrate, and I dont dose anything, only KNO3 to kill cyano algae. So you see it not needed to fert a lot.
When you use CO2 the plants grows 3-5 times faster (providing good light) so they eats 3X - 5X nutrients.
If you dont have test kit, really buy one (liquid not strip) if you will continue to dose dry ferts. I always have my test kit at hand, check Nitrates and Phosphates when something go wrong.