My tank is out of control!

when was your last water change and are these parameters before or after the water change?

Also, what type of filter are you using? Continue with regular water changes, first one should be signifigant around 40% then 10% ever other day for a week.

I would guess that for your normal maintenance you need to do at least 50% weekly water changes on your tank. If that is not being done, that is very likely the cause of the problem. The tank is very overstocked and plecos produce ALOT of waste. Water changes will be all your husband can really do to help things out! Also, if the feeder fish will not be eaten I would get rid of them ASAP!!!

pH crash?

Is that pH level at the lowest your tester can read? It may be that it is much lower than that then. Be cautious about making big changes, I'd suggest smaller water changes, maybe twice a day.

I'd also like to know what the tap water parameters are. KH of zero is a concern, that makes pH unstable. If there is low KH in the tap water you can add some baking soda to increase it. I dont know the rate, however, you may need to search for that info. Your GH is so high and KH so low, do you have a water softener on the tap water? Try to get some unsoftened water for the tank, then, but do your water changes slowly. The tap water may not have nearly that high GH and a big change there will be like opening a window in a jet plane or a space ship to the fish, very hard on the cells.

Best thing you can do is to get rid of the big feeder fish and the goldfish. That will help keep the nitrates lower when you get the tank back under control, seems to be quite overstocked if those feeders are also goldfish. If you have no emotional attachment to those, take them to the pet store for credit or for free, just to save the Oscar.

If you use the ammonia rocks because you tend to have ammonia levels, then that also indicates you have too many fish in too little water.

I suggest frequent small water changes with unsoftened water, 10% 2 or 3 times a day, increasing to 20% twice a day or so, then larger as the GH starts to be more like the new water GH.

At the moment look to do 25% water changes twice daily - one in the morning, one at night.

Remove the carbon while medicating and throw out the ammo chips - replace with regular filter media or bio-balls/rings.

Once you get NO3 down below 40ppm you're in the ballpark. Aim for a maintenance routine that will keep your NO3 below 20ppm before the last water change of the week. For a 55g with 2 plecos and an oscar, this may be two 50% changes weekly. But you want to work your way down slowly. Don't cheat by adding NO3 removing media, it just masks the problem. NO3 is an indicator of other pollutants like fish hormones, sulfur compounds, and organics which you cannot measure.

This should also solve your pH/KH issue. Test your tap water. Leave a shallow dish filled with tap on the counter overnight and test pH in the morning. The rest you can test straight from the tap. CO2 equilibrium will change the pH - that's why it needs to sit on the counter before testing, to insure air equilibrium.

Water softeners don't increase GH, they decrease it. GH measures Calcium and Magnisium. However, water softeners increase Total Dissolved Solids (TDS) which is just as important as GH, but much less easy to measure.