OK, you've got 3 major problems all with easy fixes.
Problem #1, your nitrAtes are REALLY high for fish (like to keep these below 30ppm if possible, some say below 20ppm). You need to do a large PWC ASAP, and probably a couple of large water changes. Three 50% PWC's will get you from 80ppm to ~10ppm which is much more healthy for the fish, and your plants should be fine since your Oscar and Pleco will produce a lot of waste bringing that nitrAte number back up quickly (plants prefer 20-30ppm nitrAte from what I've read). But read below first to kill 2 birds with 1 stone.
Problem #2, your pH is REALLY low. Is this during the day, in the morning, or at night when you took the reading? Since the light cycle of photosynthesis uses CO2, at night is when your pH will drop as CO2 concentrations get higher. If this is in the morning right as the lights are going on, then your not too bad, but if this is in the afternoon or at night your pH is probably even lower during the middle of the night! PWC's will help this a bit but the underlying problem is......
Problem #3, your KH is WAY TOO LOW for a CO2 injected tank. It would be fine if you weren't injecting CO2 (probably be around 6.6-7.0), but you need to be at least at 3degrees KH if your injecting CO2, especially since its a DIY as these have inherent fluctuations.
Seems to me that your tap water is very soft (this is not really a bad thing since its MUCH easier to increase hardness than decrease it). You need to gradually increase the hardness so that it can buffer the pH swings from the CO2 better. 1/2 teaspoon of baking soda will add 1 degree of KH for a 20gallon tank (roughly). I would do a 50% PWC and when refilling slowly add in 1/2 teaspoon baking soda (dissolve it in the water before adding to the tank, don't just dump it in!). This will guarantee that you are at least 1 degree of KH. If you feel this is too large of a jump (I personally don't think it is) you can start with 1/4 teaspoon to only raise it 1/2 a degree.
Wait 24 hours and then add another 1/2 teaspoon with a 50% PWC. Wait another 24 hours and add another 1/2 teaspoon with a 50% PWC.
This should have brought you down to ~10-20ppm nitrAte, and your KH should have been slowly increased from less than 1 to ~2 degrees KH. I'd then wait another 24 hours and add in 1/4 teaspoon using tank water (this will get you to 2.5degrees KH), wait another 24 hours and add in the final 1/4 teaspoon. This should have you at ~3degrees of KH which is adequate for injecting CO2.
Over the next week or so if this was my tank, I'd try to get the KH up to ~5degrees so that you have a better buffer. You may need to look into adding some calcium to the tank as well since normally very low KH water is also very low GH (general hardness).
HTH,
justin
