Just bare with me through this (there are options at the end):
The fish look small now, but they have big personalities. Cichlids are beautiful and interesting, but they have specific needs. Each one of those fish will get 5". Think about 30" of fish in there!
These fish also create a pecking order within themselves (a ranking system). To create it, they fight--it's natural. No matter what you do, they'll do that. :roll: The weaker ones will continually get chased (especially females and subdominant males--extra males who are not the "top dog"). Usually, you get a semblance of order eventually, or some of your fish die. Stocking is essential, as is picking the right tank.
Since they're in a small tank, they'll probably grow slower, but you'll be more prone to aggression. The more dominant fish (the larger ones, and the males) will feel that their territory is being constantly threatened. They'll chase, nip fins, and generally cause trouble (kill tankmates too). And when it comes to feeding, often the dom fish will keep others from eating.
The footprint is important (its surface area) for oxygen. Cichlids require lots of oxygen, and are also very dirty, which is why you need a big tank, do frequent large water changes, and overfilter your tank (by a lot). Overstocking like you are also makes your fish more prone to disease, so watch for that (excess waste--ammonia, nitrates, etc.).
As another example, goldfish have these kinds of problems all the time because people think they're OK in bowls. They're not. The fish doesn't grow, but their organs do and eventually, their bodies rupture and they die.
So, what can you do?
Prepare to upgrade, #1. I got my fish last August as juvies. They were 1". Now, they range from 2.5-3". These fish grow fast! You need to have a plan.
#2, remove the injured fish. Set up a hospital tank. If you don't have one (and don't have the means to set up a permanent hosp tank), I use home depot/lowe's buckets (5g). They're temporary, and like $3. You'll need a filter and a heater though. Hosp tanks are temporary because, obviously, they're too small. Your fish could last maybe a week in there.
Leaving the fish in your tank won't solve the problem. They might settle down, but you risk losing more fish while you wait. However, removing this particular fish won't solve the other problems, but one less fish might calm down the dom fish. You probably won't be able to re-introduce this fish though. In a bigger tank, maybe, but it's risky even then.
OR
Remove the fish that's picking on the injured fish. If there's an obvious aggressor, you could try putting him a bucket (instead of a hosp tank, we call it a quarantine/time out tank). However, another fish will take that one's place (this one might not be as aggressive though, but that's a gamble).
These fish are aggressive, vicious in comparison to tropical fish like tetras. They aren't community fish. There are some species that we say are peaceful, like your labs, but that is only in comparison to more aggressive African cichlids.
#3 up your water change schedule. Do it more frequently. Clean water isn't a cure, but it always does fish good. Since you're overstocked, I'd plan to do bi-weekly water changes, 50% each time, so you're changing all the water every week. It'll give them more air, reduce the amount of waste in the water, and hopefully reduce aggression. It might work :?
#4 check the tank temperature. Higher temps make their metabolisms faster--it makes my fish more feisty. DON'T just lower the temp thinking it will fix everything, but make sure it isn't too high. 78 is usually ideal for most species.
#5 Try rearranging your decorations. It will confuse the aggressor. Also, don't leave the light on as long. Sometimes, leaving the light off can calm them down--I've heard. However, my fish do most of their fighting "at night" ie when the light's off. SO, that could go either way.
#6 make sure you have lots of hiding places--not just a plastic plant or a sigh post in there. :lol: Get some rock, build a structure/pile of some sort. Try to get two piles that don't touch in the middle. The dom fish will pick one, and hopefully the outcasts can share the other. This gives your weaker fish places to escape to.
Just some things to try.
When I first got my fish, I didn't know about footprint either. I got a 40 breeder (3ft) tank which I thought was a good size. I put (5) y. labs, one socolofi, and one red zebra in it. My female red zebra quickly ate the socolofi, but, and I have no idea why, gelled with the labs. However, since I put in lots of rock for them, that displaces water (less water, less air), so be aware of that. Within 3 months, I upgraded to a 55.
I don't know if you have a petco near you, but they routinely do a tank/gallon sale--you pay $1 per gallon for the tank. Just the tank, but still a 55g for $55. Or check craigslist. There are always ads up there.
You're doing the best thing for your fish right now--you're researching, asking for help, and you're caring about their welfare. I know so many people who don't.
(I have a friend with a bumblebee in a 5g tank--makes me absolutely cringe. I've tried to tell her this stuff, but she's uninterested. She cares more about herself than the fish. Those people shouldn't keep fish, you know?)