Need Aquarium Help, Please

Hey All,
I have a 55 gallon aquarium inhabited by 1 7-8 inch Oscar, 2 Jack Dempseys (1@5-6 inches, 1@4-5 inches), and 1 12 inch or so plecostomus (nevermind the spelling).
I use a powerhead to circulate water, and a filter thingie that hangs over the back (assured by the pet store at time of purchase this was the proper size for the aquarium).
I have 2 inches of large gravel on the bottom of the aquarium- at least I did before the oscar decided to rearrange it mostly into the corners, and bare in the middle. I have a pseudo branch thing in the aquarium also. The setup has not changed in 7 months.
I feed the fish 2 dozen or so goldfish or 3 dozen minnows 1-2 times per month, with pellets a few times a week, and the occassional algae wafer for the bottom feeder.

THE PROBLEM: Green water. The color ranges from light green sometimes to “can’t see the fish unless they are right against the glass” dark green. It kind of cycles from bad to worse and back.
Sometimes, after a feeding, the water smells, so I do a 1/3 or so water change with water that has sat out for 24 hours. I also change the filter cartridges at that time.
Now that the sun is out more often, I am getting a bit of algae on one small side (the aquarium is in a corner, out of the sun, but if I open the blinds, it gets a little direct sun on one small side for a few hours in the afternoon).

Any idea what the green is, and how to get rid of it?


The green in the water is algal blooms feeding on dissolved nutriants in the water. The fish you have are all large, active creatures which put out quite a bit of nutrient-rich waste, especially when you’re feeding the Oscar and Dempsies on live food on a regular basis. Your filter isn’t doing anything more than pulling particulate matter out of the water stream and giving bacteria a place to live; the nitrogen and phosphorous compounds in your water that come in from the food you’re putting in don’t have any way to be removed from the tank other than through water changes.

First thing you can do is more water changes - 10 gallons of water change a week should get rid of the algal blooms. You also need to get yourself some test kits so you can directly measure ammonia, PH, Nitrate, and phosphate levels of your water. The tap water you use for water changes can be another source of phosphates and nitrates, so you’ll want to check that.

Live plants would help keep nutrient levels down, but the fish you’ve got in that tank would just rip them to pieces. Fundamentally I suspect you just have too many fish in too little of a tank.

I have a 29 gallon setup that is currently empty (the Oscar’s home when he was very small). If I move the smaller Dempsey over and cut back on the live food do you think there would be a significant improvement?

My other inclination is to purchase another 55 gallon setup, and make that the new home for both Jack Dempseys, but I really don’t know where I would put it…

What you really need to do is get a handle on how bad the problem is. Buy some test kits and find out exactly what’s going on with your water chemistry before you decide how to fix it.

Here’s a good website, and a good message board:


I agree with Andrew–buy a water test kit to find out more precisely what the chemical situation is.

Oscars, Jack Dempseys, and other South American cichlids like acidic water–and are also high producers of waste. (Your tank sounds very similar to my old 60gal–2 7" Oscars, breeding pair of Dempseys, pleco, etc.) With such large fish and their considerable waste production, the nitrite/nitrate cycle can get thrown for a real loop. The green could be algae blooms, but it could also be natural bacteria going ape. Buy a kit that tests for nitrites, nitrates, and ammonias; they’re not too expensive and valuable information providers.

Depending on what the kit tells you, you may indeed want to move the Dempseys over. Even so, I wouldn’t think that’s necessary if you start with 1/3 water changes every other week or so, plus changing the carbon in the filters.

Good luck!

We fought with it for ages. Finally we started leaving the aquarium light off except for 1/2 hr at feeding times. Otherwise, the tank gets ample room light. The aquarium light was just making the alga grow like bashees.

Haven’t had trouble in 2 years now.


Someone suggested changing at least ten gallons of water per week and that is an excellent suggestion. For cleaning the inside glass walls, use nothing but nylon mesh. I think you have two many large fish in one tank.

And the water you drain out of the tank—feed it to your houseplants: You will see some really great results.

Personally I don’t think you have too many fish for the size of aquarium, but I think you need a better filter, regardless of what the pet store/fish store people told you.

I would recommend a biological filter - mine are fluvals and they do a rather stupendous job - my big one circulates about 350 gallons of water per hour, so the tank water is completely recirculated about 5-6 times an hour. (I have a 66 gallon).

Secondly, I would try an algae treatment - I like Fritz “Algae Clean Out Liquid” - does a decent job and didn’t kill my Pl*co.

Keep up with the water changes as previously recommended, and turn the temperature of your tank down a little. Buying a test kit is a good idea too, however neither increased nitrates or increased ammonia will result in the conditions you describe - the only way I know when somethings funky with my water is when the fish start doing barrel-rolls. Sevrums are just not supposed to swim upside down. :smiley:

FYI, I currently have a 66 gallon with 1 14 inch pl*co, one 8 inch sevrum, a 7 inch dempsy, two 5 inch jade eyes and an 8 inch cobalt, not to mention a shwank of feeder fish. I’m currently in the market for another cichlid of some sort - if anyone can recommend something nice and a bit different, I would be interested - it has to be hard core though - my fish are pretty nasty. Blood parrots are really pretty, but I have a feeling they would be lunch sooner rather than later.

I also have a 33 gallon with angels, platties, mollies, tetras, etc, etc, and a 5 gallon with zebra fish. Both the 66 and 33 gallon tanks have biological filters on them and they have MUCH less problem with algae overgrowth than the small tank which has a hanging filter like the one you’ve described you have. (Holly run on sentence, BatMan!)

Anyway - Good luck!
:smiley: :smiley: