Fin rot and can't get nitrites down

[pumpkinsmum]

Hi, I've had a 3inch ranchu in a 10 gal (I didn't know tank was too small when I first got him, planning on getting bigger one when circumstances allow). I didn't know about cycling process so I did end up doing fish in cycle. I tested water obsessively during cycle and around 1.5 to 2 months in I had nitrates, no nitrites and no ammonia. Was stable for probably a week after that. Then I stopped testing as much and didn't change water as often, we're on month 3 now and I tested the water and was shocked by how high nitrites were. After a week of doing daily water changes (around 30%) he has developed fin rot on his tail and I'm really worried about him.

Perhaps I am overfeeding?
As weather has gone colder I've started to use hot water out of tap when doing water changes when previously I only used cold, is this the cause?
Perhaps I just need to do more regular water changes once it's stable?

Any advice welcome thank you

 

[Aiken Drum]

What are you considering high nitrate?

You are going to get high nitrate keeping goldfish in such a small tank. They produce a lot of waste relative to size of fish, that waste converts to nitrate through the nitrogen cycle. You should probably be doing twice weekly 50% water changes under normal circumstances. If you have allowed nitrate to build up, this may take a lot of big water changes to get it down again.

Do you know if your tap water contains nitrate? Do a test on your tap water, or better still, check with your water company, they will do more accurate tests than your home test kit can. I notice you are from UK. Your water test records should be available online. UK allows up to 50ppm nitrate in tap water.

 

[ColdKoi]

Is it nitrites or nitrates? I’d assume you meant nitrates and regular water changes are the only way to keep them in check

If you’ve got nitrites you could have a cycle that has crashed or the filter is not capable of housing enough bacteria to complete the cycle to nitrate.

What type of filter do you have and what kind of maintenance (if any) have you done in the last month?