The dreaded green water...
Green water (what you're suffering from) can be tricky. I've had several battles with it myself. Here are some options that will fix the problem. All have their merits and drawbacks:
1. Bleach. Remove your fish. Pour bleach into your tank. Wait 15 minutes. Algae is dead. The problem with this method is that everything is now dead, most importantly your biological filter. Thus, if you use this method, properly dechlorinate, then foolishly return your fish immediately (as I've done), you're placing your beloved fish into an uncycled tank. I've used this method with some success on an overstocked goldfish tank, but it is not something I would recommend.
2. Filter. To my knowledge, the only filters capable of removing suspended algae are diatom filters. I have a Vortex diatom filter myself, and the thing is just amazing. It still surprises me how quickly the thing cleans up my 90g after the occasional outbreak. Drawbacks? Initially, price. You gotta buy the thing. Cleanup is also a bigtime hassle. Lots of websites will warn you about this, and it's even worse than they suggest. If your tank is large, like mine, it may take three 1.5 hour runs of the filter to clear it up. That means 3 times spent cleaning out the filter (which takes anywhere from 5 to 15 minutes, but is really messy and obnoxious). Plus, you really haven't solved the real problem, which leads me to number 3...
3. Solve the real problem. I've seen lots of suggestions as to what leads to green water. There is actually spirited debate over it, but it basically comes down to two things: too much light or a chemical imbalance (nitrates and phosphates are the usual suspects). Unless you fix whatever is causing your algae bloom, it will keep coming back. Again. And again. And again. How long are your tank lights on every day? 12 hours? Then try 8 or 10. As long as there is a reasonable amount of ambient light in the room the fish don't need them anyway. What is your water change schedule like? Maybe you need to change more (this is almost certainly my issue when green water creeps up on me). How much filtration do you have in the tank? Maybe you need to add more. The more you turn that water over every hour, the better (for a 55 gallon, you should probably be filtering a minimum of 150 or 200 gph)! The drawback to this method? It requires a tremendous amount of patience! Your green water will not disappear right away. However, I assure you, if you fix the root of the problem, your water will end up crystal clear.
If money isn't an issue, give your tank the ol' #2 followed by the ol' #3. If money is an issue, just cut those lights back a bit and increase your water changes. I have a 90g stocked with two Oscars, a Jack Dempsey, and a Plecostamus. If I don't keep up on maintenance, green water is guaranteed. You say you have S.A. Cichlids, so I suspect you have some big fish as well.
I certainly haven't listed ALL of the options, only the ones I have personal experience with. Perhaps someone else can shed some light on UV sterilizers, though I've heard those can be quite expensive and problematic.
One final note: have no fear, green water is totally harmless! Your fish don't care. I'm actually pretty sure my Jack Dempsey preferred it...
Hope this helps, and best of luck!
Cheers.
-sj
