Loaches Online Forum

I always do 90% water change every week, but this time i do a nitrate testing. Before i change the water i tested it was 40+ppm nitrates. After water change it went down to 5ppm. Next day i tested it went back up to 40ppm. So i clean up my sump filter completely change a new pad and clean bio filter with tap water, put in a new purigen bag. After only few hours i tested again it went down to 10ppm nitrate.

If i clean up filter every week, i can skip water change? maybe i can do water change once a month instead of every week?

No

Cleaning your sump was most likely long overdue, and from that I would suggest cleaning it once every 6 or 8 months

However, what the sump basically does, is turning waste into nitrates - and if the sump does have a lot of waste, it will produce quite a lot of nitrates.

I would, however, not change 90 % of the water in 1 go - I would not change more than 50 %

Changing 50% water is not enough for my system. My nitrate is very high everyweek 20 to 40 ppm. Sometime i do 98% water change when my nitrate hit 80. I always overfeeding. I know im not suppose to, but i cant help it. 80 small clown loach go crazy at food is an amazing sight.

I prepared 6 brute trashcan 50 gallons and a heater in each before a week ahead for water change. I been doing this over a year now. My fish is very happy and heathy after the water change. I can see the different the next day.

Your system was polluted by the dirty sump, which is now clean.

Therefore it will not be polluted as fast as it used to be. However, due to the cleaning of the sump, it will, for a while, not work as efficient as you would prefer.

In fact, were it my tank, I would feed the fish: nothing this week, and only a very little next week

Perhaps, consider doing 50% change twice per week. Safer.

Cleaning your mechanical filtration regularly is a good idea. This allows you to remove detritus before it enters the nitrogen cycle.

Dissolved organic carbon can also inhibit nitrification (along with low alkalinity, low dissolved O2 levels, etc.).

I once had low levels of ammonia I couldn't get rid of. I was barely feeding the fish and doing large daily water changes. I had no nitrites or nitrates. I eventually checked inside the built-in overflow and found a thick layer of sludge. I removed a lot of sludge from the overflow and also the bottom of the sump. Nitrification started working again in that tank.

You need to do both: Water changes, and cleaning the filter.
The filter needs to be cleaned often enough to keep the water flowing freely through all the media, not escape around it when it is too choked with waste.
Try cleaning the filter once a month, and maintain your same water change schedule, and see what happens to water parameters.

You are right, though, that the more nitrogen you add to the system (protein in fish food adds a lot of nitrogen) the more you need to remove.

Live plants can do this, and there are several ways you could use them to help.