Clown plecos

Is it bad that I'm keeping a clown plecos in a ten gallon aquarium?

Yes, but is it temporary, or is this the only tank you have? This fish attains 3-4 inches, and like most plecos can produce a lot of waste. A 20 gallon is the minimum tank for one, and that would take regular maintenance (water changes, vacuuming substrate).

Just in case you are not aware of the diet of this pleco: In its habitat, this fish feeds on logs and branches, eating the wood itself along with aufwuchs [algae mats containing rotifers, small crustaceans, insect larvae, protozoans]. In the aquarium wood must be provided. This fish will eat some species of algae, prepared sinking foods that contain vegetable matter along with shrimp, fish, etc. Blanched vegetables such as zucchini, yams, squash, cucumber, spinach, romaine, banana. Live or frozen brine shrimp, bloodworms, blackworms.

Here is a good article on this fish: https://www.planetcatfish.com/shanesworld/shanesworld.php?article_id=355
Here is the Planetcatfish page on the species: https://www.planetcatfish.com/common/species.php?species_id=734

If you are keeping only a single or pair, then a 20 long would work. The above article features a breeding group of 7 in a 40 breeder.

In days gone by I have kept a pair of breeding bristlenose in a well planted 10 gal. I did not particularly want offspring but I got lucky because I had amano shrimp in the tank and they can and do eat tiny fry.

Not only are more gallons helpful with keeping plecos from a waste point of view, but also, because they are territorial, area matters more than height, hence the 20L suggestion above rather than a regular 20.

It was not clear to me how many clowns you have as you wrote "Is it bad that I'm keeping a clown plecos in a ten gallon aquarium?" A pleco is a single but you wrote "plecos" after the a. So did you mean "a pleco", or did you mean plecos which would indicate more than one?

I meant just one and he's just a little guy about 1.5 inches. And this is the aquarium he lives in.

That is a nice aquarium. Unfortunately it is small long-term for this fish. But if there are no others, and you are regular with water changes, it might work out. However, the wood is a serious issue, as this pleco needs real wood to graze. The decor looks like rock. If you could find a chunk of aquarium bogwood, it would provide what it needs.

There is some wood buried beneath the moss.