Clown Loach is sick

[Granny loves fish]

I have 4 clown loaches that are 5-6 years old. They are in a 120 gal tank that is well established. I only have about 20 fish in the tank and do a water change weekly and clean my filter 1 time per month to month and half. I just did a filter clean and water change. I only check my nitrates and they were at 4 a couple days ago. I have some live plants in tank. There are no other sick fish and I haven't lost a fish in quite some time. Not sure what to do except do small water changes daily to keep the tank very clean. Any other suggestions as to what I can do. I do not have a quarantine tank as I haven't had a sick fish in some time. Obviously time is of the essence, if I can do something to save this fish I'm sure I will have to act quickly. Thanks for your help

 

[fluttermoth]

Could you give us some more details of what's wrong with the fish, please? What symptoms is it showing?

How, exactly, do you clean your filter, what percentage of water are you changing, and what are you using to to test the water; I've never seen a nitrate test with '4' as one of the possible results!

What species are the other fish?