HELP!! Catfish fighting?

I have two sucking catfish in my 20 litre tank. One of the sucking catfish always seems to annoy the other by sucking on it or chasing it around. It makes my catfish ( the one getting bullied ) very stressed and I need a quick solution before it gets too stressed and dies.

The grey one if the one bullying the white one. Its like he always wants to be near her.

Thanks, any suggestions would be highly appreciated.

Two things, one, those are sucking loaches (Gryinocheilus aymonieri) they are aggressive toward each other. It makes it worse that you have them in a 20liter tank ( 5 us gallons). Sucking loaches grow big especially for a 5 gallon tank. They grow to be about 8 inches (208mm). I would relocate them into separate tanks until you figure out what to do with them.

Ahh yes, the terrible Chinese algae eaters. They are not nice and grow too large for a 5gal anyways. They will attack other fish as well as each other.

Honestly I feel those should not even be in the aquarium trade, just like common plecos. Too many people buy CAE when they are small thinking they are cute and will eat all their algae, when in reality they become monstrous and often attach their tank mates. I've made the mistake once, never again. The rotten thing was a terror.

If you can, I'd return them. If you can't then get a larger tank for them (if you can). Nothing much will solve the aggression issues as they are confined to a small space, unable to get away from each other even if they wanted to.