green algae bloom in my established tank

Question
I have a 55 gallon aquarium where I house my tiger oscar.  The oscar is about 3.5 to 4 inches long.  It gets feed a small amount of food once a day and the tank is vacuumed along with a 25 per cent water change once a week.  This tank behaved just fine for about 6 months.  Last October my family and I moved, and so the tank was hit with a new municipality's water, but even that seemed to have no ill effects for about two months.  Now to the problem.  For the time until last Christmas the oscar had a large pleco for a tank mate.  Over Christmas, while we were gone visiting family for a week we lost power for about 2 days.  The oscar was fine but the pleco didn't make it.  How long the dead pleco was there I'm not sure, but it was too large for the oscar to consume any of it.  At any rate as soon as possible the dead pleco was removed and a water change (about 50 per cent) was done with gravel cleaning.  Soon after the water went green.  Very green.  We have tried moving the aquarium to a less sunny part of our house (it now gets no sunlight), we have tried covering the tank with garbage bags for up to five days at a time and we have tried the products P clear and B clear.  Nothing has worked at all except the last two chemical products.  However, after a couple days of clear water it goes right back to green again.  And we are talking liquified Incredible Hulk here.  As of right now if my oscar is at the back of the tank (its a long tank, only about a foot thick) I cannot see it.  Not even a shadow of it.  What should I do.  I've talked to five different pet stores and have gotten five different solutions, none of which have worked.  I'm exacerbated here.  Please help!

Answer
Hi Clinton,

Green Algae water is caused by too much nutrients in the water. Try doing some 20% water changes daily. This will remove most of the nutrients that algae need to grow. There's a possibility that your tap water has too many nutrients (nitrates, phosphates).

You might want to purchase one of those small at-the-tap filtration units that hook up directly to the faucet. These little things are available at any grocery/supermarket, as well as hardware stores. They can filter out impurities in the tap water.

If that does not work, get a phosphate removing agent. This can come in the form of a filter insert, like Hagen's GreenX, or a chemical to put in the water. The phosphate remover combined with partial water changes will effectively 'starve' the algae of all nutrients.

One last option would be to get live plants. If you do not have plants, this would be a good opportunity to get them. Plants will compete with algae for nutrients and light - I've never experienced green water/algae in tanks with plants! Choose nice, sturdy and hardy plants that can stand up to your curious Oscar.

You've tried a lot of things, and I believe that the problem may lie in your water. If you can... maybe get another Pleco or other algae water, since this seemed to keep your algae in check.

Good Luck, and Happy Fishkeeping!