Hi and welcome to the forum
How long has the tank been set up for?
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Is the suckermouth catfish an actual Plecostomus or is it a Bristlenose catfish?
Both are referred to as plecos but one grows to 2 foot long and the other reaches 4-5 inches long.
If you can post a picture of the fish we can identify it for you. If the pictures are too big for the website, set the camera's resolution to its lowest setting and take some more. The lower resolution will make the images smaller and they should fit on this website. Check the pictures on your pc and find a couple that are clear and show the problem, and post them here. Make sure you turn the camera's resolution back up after you have taken the pics otherwise all your pictures will be small.
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What are the tank dimensions (length x width x height)?
What is the GH (general hardness) and pH of your water supply?
This information can usually be obtained from your water supply company's website or by telephoning them. If they can't help you, take a glass full of tap water to the local pet shop and get them to test it for you. Write the results down (in numbers) when they do the tests. And ask them what the results are in (eg: ppm, dGH, or something else).
Depending on what the GH of your water is will determine what fish you should keep. Tetras, barbs, gouramis, rasbora, Corydoras and small species of suckermouth catfish all occur in soft water (GH below 150ppm).
Livebearers (guppies, platies, swordtails, mollies) occur in medium hard water with a GH around 200-250ppm.
If you have very hard water (GH above 300ppm) then look at African Rift Lake cichlids or use distilled or reverse osmosis water to reduce the GH.
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All tetras naturally occur in groups of thousands out in the wild and should be kept in large groups in aquariums. A minimum number for any species of tetra is 6 and they do much better when kept in groups of 10 or more.
Black widow/ skirt tetras are fin nippers and will chew the fins on any fish that is slow moving or has long fins.
Black phantom tetras are lovely peaceful fish. The males have a bigger dorsal (top) fin and black pelvic (little fins under their belly). The females have red pelvic fins.
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Because you have black widow/ skirt tetras, it will make it more difficult to find compatible tank mates. And if you have a genuine Plecostomus catfish, that will cause problems later on when it grows.
Several options are available. Remove the black skirt tetras and get some more black phantom tetras and some cherry barbs or ember tetras. Alternatively, keep the black skirts and get rid of the black phantom tetras, then get some serape tetras, which are also fin nippers. Make sure you get a group of 6 or more of each species.
Personally I would go with black phantom tetras and cherry barbs and get rid of the black widows.
Then you can work out what you want to do with the pleco.