Concern in my 75-gallon community tank

[Stefan]

Hey all,

So I've got a 75 gallon tank with a pretty diverse group of fish. There's my red spotted severum, an EBA, a Synodontis catfish, three Rainbows (down from five, sadly), and six polar blue parrot convict cichlids.

I've been doing 40% weekly water changes, and the syno's been doing a great job keeping the nitrate levels in check. The water readings are perfect, but I still lost two Rainbows recently. One had lymphocytis from the start, so I figured that was the cause, but then another one died a few weeks later. I've been observing the tank, and I noticed one of the Rainbows doesn't school with the others. It just hangs out alone, but it doesn't seem lethargic or sick.

I also noticed that when the Rainbows get too close to three of the bigger PB cichlids, they get chased away. It's not aggressive, just a quick nip to keep them at bay. This is new behavior, and I'm wondering if the stress from the PB cichlids might've contributed to the deaths. My theory is that the PB cichlids are stressing out the Rainbows, but they're not actively targeting them.

I'm considering a few options to address this. I could add more tall plants to break up the tank, but I'm not sure that'll make a big difference. I could remove the three larger PB cichlids, but I think the smaller ones might just take their place. I do love the PB cichlids, though. Another option is to rehome the Rainbows and try introducing something new, but I'm not sure what would work well in the tank. I've tried other fish in the past, like rainbow cichlids, acaras, geophagus, and gouramis, but they didn't do well.

I'm also thinking about bringing the Rainbow population back up to five or six to see if that'll help. Maybe I could remove the most aggressive PB cichlid and rearrange the tank to create new sections for the fish. Any thoughts on what I should do?

Update while cleaning the tank - I've been observing the behavior of my 3 remaining rainbowfish, and I think I might have found a potential cause for the deaths of the other two. The one orange rainbow that lingers by himself seems fine, no marks or anything, but the other orange one and the blue one that travel together have some noticeable fin nipping going on. Specifically, the blue one is nipping at the orange one's fins, and I can see the damage. This got me thinking - the two rainbowfish that died were also orange, and I'm starting to wonder if the blue one might be the aggressor here. Could it be that the blue rainbowfish has been bullying the others to the point of killing them?

[coltin]

Rainbowfish generally don't kill each other, they just show off and display to one another. Sometimes their fins might get slightly damaged in the process, but it's not usually a big deal. There is an exception, though - big male Glossolepis incisus can be bullies, and occasionally they'll kill smaller males or females. This tends to happen when there's only one big dominant male and a couple of smaller fish. I've never had it happen in my tanks, but I've always kept a lot of rainbows together - 10-30 or so, all similar in size. Others have reported problems when they've kept a large male with smaller fish, like a 4-5 inch male G. incisus with a couple of 2-3 inch fish. The males get territorial and start bullying the smaller males until they eventually die. This kind of thing doesn't happen in the wild, where there are plenty of fish and lots of space to swim away from any aggressors.

[coltin]

I'd love to see that video, if you can get a clip of them biting, upload it to YouTube and paste the link here, that would be super helpful.