Okay well to be completely honest I got these fish not knowing much about how to take care of them. I've now found there's a lot more to it and I don't know how to fix any of the levels. There all not really where there supposed to be. I have one anglefish and one pleco in my 20 gallon tank. I tested and found general hardness was high as it could be, and same with nitrate. The nitrite was at 0.5 and pH was 6.0 and carbohnate hardness has been at 0-40. Not sure how to test ammonia though. I tried uploading pictures but it says there to large of a file to upload.
It may help to explain the difference between water parameters and water conditions. Parameter refers to GH (general hardness), KH (carbonate hardness or Alkalinity), pH and temperature. Conditions refer to ammonia, nitrite, nitrate, and any other things that may affect the water quality.
Taking parameters first. If the pH is around 6 that is on the acidic side, and well suited to soft water fish species (angelfish and pleco are soft water). KH between 0 and 40 is low, which makes sense. The GH should be similar, as these three are closely related. It would help to know the number for the GH. You might be able to find this out from your water authority, check their website. The GH and KH will normally remain close to what they are in the tap water; I will leave it at that. The pH may lower naturally due to the low GH/KH if this is the case. You do not want to be attempting to adjust these parameters; this is possible, but can become very complicated, and with fish in the tank these values should remain stable and not fluctuate around as the fish can be impacted. Once you know the parameters of the source (tap) water, work with that and select fish suited.
As for ammonia, nitrite and nitrate. You can buy test kits for these. Many of us here recommend the API Master Combo liquid test kit; it has tests for these three plus pH, and is basically all you need moving forward. Ammonia and nitrite must remain zero with fish in the tank. I suspect the nitrite was during initial cycling. What is the test number for nitrate (you say it was high, but how high?).
On the photos, you can downsize photos on your PC. I am not technical, so won't get into how.