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I've aquired a featherfin squeaker. A guy brought him into the PetSmart my friend works at, and she immediately asked me if I had a place for him(don't actually know the gender). I have a 75 with cories, amanos, and some neons, guppies, and a female Betta. I also have a 125 with large tetras, botia kubotai loaches(3), and 3 clown plecos. The clown plecos and loaches get along great, but I'd worry about adding the new guy. However, it might be an option to move the clowns to the 75 with the cories and add more driftwood for hiding. I'm afraid to add the squeaker to the Cory tank because he's pretty big already (5ish inches). Looking for helpful suggestions. Thanks.

My Featherfin is the oldest of my Synos and while he pushes my other Synos around, he ignores my plecos and loaches. And he only squabbles with the synos the same size (my hybrid and my pleurops) or bigger (my decoras). He ignores the smaller plecos and the pictus cats.
So I am thinking even if he is the biggest dude in the tank that unless you put another syno like him into the mix, that things should be ok.

He might eat your shrimp and neons though, so the bigger tank is probably better. Mine has never eaten other fish, but given the opportunity of a tasty little snack he might indulge...

It looks like in general eupterus are a hit or miss, a gamble, which makes it impossible to predict for sure what will occur when one is added to an established tank and community. They can prey on tiny fish like your neons but don't always do it. Yet it is usually considered a significant risk. They are usually more or less ok with other tank mates but not too seldom problems of various severity have been reported by hobbyists, from mild scuffles for hides or territory and hierarchy enforcement to outright murderous practices like rasping slime coat off of tank mates.

What I am leading to is that IMHO the general recommendations appear somewhat wishy-washy and often one has to experiment and be prepared to change things around or adjust or even remove and rehome. If you cannot stomach possibly upsetting an apple cart (the chance is small but not zero), probably you shouldn't gamble... albeit in most cases I think eupterus fit in more or less free from major problems.

Like you said, adding driftwood, plants, furniture, etc. can alleviate the problems but you will see less of your fish too reducing aesthetic pleasure and also may not catch ongoing problems in time.

I have 4 and they eat anything smaller than an inch long in the tank. The biggest is about 5" So I would not trust them with neons or shrimp or anything else small.

My featherfin is the oldest of my Synos and the undesputed king of the tank. It usually envolves a bit of chasing (he owns the left side of the tank) and some pushing. It is reserved for the ones around his size or larger (my pluerops, decoras, and hybrid), he ignores the smaller ones. He also ignores all the other catfish, plecos and loaches I have had him with.
I would guess just due to their size itty bitty fish would be seen as a snack (finally figured out what was happening to my penguin tetras in another tank when my biggest SA bumblebee choked to death ) as well as shrimp.
I would not say my big guy is aggressive. He just has his ideas of how things should be and where the rest of his tankmates belong .