A water change once a month is nowhere near enough. The toxins (primarily nitrate) slowly build up, and the fish get used to it. But it is a low level stress. Then something else happens, and that low level of stress, going on so long, has left them with less recovery potential and they die.
A filter change throws away the beneficial bacteria that have been growing on the filter media. There are still some bacteria living on the other surfaces in the tank, for example on the substrate, on decorations, on plants and other things. Still, that loss of bacteria from the filter media allows ammonia to spike. This may or may not be a problem. If the pH of the water is low (under 7.0) then most of the ammonia will be in the form of ammonium (NH4) which is a stress, but not as deadly as ammonia. If the pH is over 7.0 then the ammonia is in the form of ammonia (NH3) which can kill the fish pretty fast. Older tanks with insufficient water changes tend to have a lower pH, but not always.
An ammonia spike is often followed by elevated nitrite levels. This is because the bacteria that remove ammonia grow pretty fast, so convert the ammonia into nitrite. But the nitrite removing bacteria are slower growing, so the nitrite lingers. Read about Brown Blood Disease to see how toxic nitrite can be.
Other things could have happened, too, and we have no way of knowing.
Suggestion: Give away the lone Clown Loach.
Get a test kit. Read up about water chemistry, and what happens with insufficient water changes.
Do the fishless cycle while you are researching fish. This will keep the beneficial bacteria alive. (Fishless cycle is at the top of the Health forum)
When you are ready to restock you will need to do a very large water change to remove the nitrate that will build up during the fishless cycle.
Fish ideas for a 55 gallon:
Maximum adult size: 6", and that only if it is a fairly sedentary fish. 4" for an active species.
Get something that lives in the upper area, perhaps a single Gourami, or a school of Hatchets. (Covered tank!)
Get something that lives in the middle area, perhaps a school of Tetras, Rasboras or Barbs.
Get something that lives in the lower area, Loaches or Cories. A good Loach for this tank might be the Zebra Loach.
If you want to stick with Asian fish, then...
(1) Pearl Gourami
(15) Harlequin Rasboras
(6) Zebra Loaches
Happy fish keeping!
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