Originally Posted by
dspeers
Siphon 3 gallons from your aquarium into a 5 gallon bucket. Remove your HOB filtration and rinse off in the bucket, you can reuse the sponges after cleaning, in fact you want to to maintain your nitrification capability, that's why you don't rinse in tap water. If the water is heavily fouled you have your answer. Just like nitrates will tell you how much/often to change your water, how cloudy the water is tells you how often you need to clean your filter elements. You also need to siphon clean your substrate and see how much crud has accumulated there, you might be stunned.
You simply do not have enough fish to explain your nitrates, therefore you either
1. Have a bad test
2. Are adding too much crud aka overfeeding
3. Not getting rid of the previously processed food, aka fish poop in the substrate and/or filter that bacteria will break down to nitrates and phosphates (unless you missed a deceased fish, absolutely wonderful nitrate source)
Biochemically bacteria "eat" detritus and generate nitrates and phosphates, in addition carbon metabolism generates organic acids and CO2, all of which drop pH so I also agree with more frequent less volume WC, understand that 25% every 12 hours removes the same nitrates as 50% every 24 with less pH fluctuation. Have you been following your pH changes just before and then 30-60 min after water changes? You definitely want to solve this because a group of 5 discus is better if you want them happy, unless your goal is a breeding pair.
Finally since we are discussing water quality, you also need to measure your carbonate hardness as kH measures your water's buffering capability aka ability to absorb acids without wide pH swings.