from your description it sounds like a DOJO loach or WEATHER loach.
Catfish eat using there mouth. Most are bottom feeders. They usually gulp or suck in gravel, take everything they want off it and spit out the gravel.
Catfish... aren't picky in the slightest. It wound be like asking a pig if it would rather live on a farm or an orchard. You can have a bare-bottom tank and they will be happy. As long as there bellies are full, it doesn't matter. but keep in mind. Large catfish LITERALLY inhale their food. So if you have large catfish, it be best to have gravel to prevent choking.
it could be, it is always best to have at least two animals from one kind in a tank(there are exeptions) NO, it is a solitary animal and that's what it likes to do.
I have two Bristlenose catfish and they are doing a great job of keeping the tank crystal clean. I would definitely recommend these.
filters only filter the water they don't clean the walls of the tank or the gravel in your tank. you still need to move rocks and plants to get to the crap left behind that the filter cant get. you should do this when you clean your filter. depending on what sort of fish you have they can clean the tank for you cichlids will definitely clean the gravel for you as they build nests like birds and are always sucking up rocks. you should try to get a catfish as these are bottom feeders and they will stir the water up for you as well. you can get some catfish that will go good with goldfish as well.
gravel
Gravel is Inorganic
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gravel
Mike Gravel's birth name is Maurice Robert Gravel.
Tiff Gravel's birth name is Tiffany Reagan Gravel.
abiotic