algae bloom

Question
I have a 150 gallon tank with a 10 inch oscar and 1 pleco nearly the same length.  The tank was set up about 2 years ago and it has a large piece of driftwood and plastic plants.  It is located in a storefront office where there are lots of windows, but the tank is behind a cabinet and does not get direct sunlight.  I am currently running a Rena cannister filter (for up to 265 gallons) and a Marineland magnum 350 filter.  I do regular weekly water changes, usually at least 45-50 gal.  The water will clear up for a day or two, the turn so green you cannot see into the tank at all.  I have had to scrape the sides and remove decorations in the past, but it is not accumulating on surfaces anymore since I got the pleco in June.  The green water will not go away though.  I have also tried every brand of chemical algae killer there is....none of them work.  I was told that my water may have phosphates, so I have had a phosphate remover pad in the filter for 2 months.  The lights used to run on a timer 8 hours a day, but they have been off for 2 months.  I have also tried draping the tank from Thurs afternoons to Monday mornings.  I have tested the water on my own and had 3 different fish stores test it.  All levels the kits test for are fine.  Last week, I installed an 18 watt uv sterilizer (Coralife turbo twist).  I have changed out about 60 gallons since then, but the water is still very green.  Last week you couldn't see into the tank at all.  Now, you can tell where the decorations are, and see the oscar IF he happens to get close to the glass.  

Is the flow rate too fast, or am I doing something else wrong??  How long should it take for the uv to clear it if it is going to?

Thanks for your help.

Answer
Hi Melissa,

Your algae bloom is because you use a canister filter. They are nitrate factories.  Algae feeds on nitrate.  When you change the filter and the water, you stir up the bad stuff and the algae feeds off of it.  Any light will be a source to make it grow.

When changing the filter from now on, wash the inner canister completely.  Wash the media also.  Try to get the dissolved waste out.

There could be another cause.  Dirty gravel.  Have you ever cleaned the gravel?  Gravel should be siphone cleaned monthly.  

The lights should be lowered to 6 hours at this point, maybe even 4 until the algae growth diminishes.

You may consider adding more filtration.  Canister filters need weekly cleaning.  Hob filters at least bi weekly.

I hope this helps and if not, write back and let's go into more depth.  My questions for a return on this answer would be the following:

What are the nitrate levels? Nitrites? Ammonia? Phosphate? Are you using tap water? (If so, please stop and stop using chemicals - get some over the counter water from a grocery store refill station in 5 gallon water containers.  It's highly filtered and UV sterilized in some cases and it costs about a buck per five gallon bottle and your water is guaranteed not to be hard, or soft or have phosphates from that station.  Also, you need not add any chemicals to it).

What is the filter-changing regime?

How much water do you change at a time?

What kind of food are you feeding?  How much?  How often?

Have you tried a water clarifier product?  If so, this is a major source as to why your water is so green.  Water clarifiers clear the water by putting small phosphates into it.  They bind the floating matter into small balls.  Filters carry that into them and it turns into nitrates.  That turns into a nitrate factory in the canister if not cleaned weekly, as recommended.  The cycle of severe nitrates caused by using clarifying products and canister filters is a cycle many aquarists encounter.  I'd like to see you change from doing that by trying the filter-station water and beginning with clear water completely.

Renas are up to 265 gallons IF they are maintained weekly.  When they are not, they are not that high.  People think they can hook them up and forget them.  Weekly, bi-weekly at the least, clean them and watch the water improve.  

I use the same Marineland you do, but it's on a 20 long.  They are great filters, if used appropriately.  They are rated from 30 to 50 gallons.  I would say go more with the 30 than the 50.  

I hope this helps, but if not, I'm here, and I will absolutely try my best to help you through this tough time and to find appropriate answers.

Sincerely,
Renee