Hi Beverly,
I take your referring to surface algae and not algae suspended in the water column?
Is your tank planted?
Yes, it could be the tap water is a root cause.
Lets look at what I know, and what I've used successfully in the past.
Note: this is for various algaes, not cyanobacteria, cyano is a whole different subject to tackle.
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Algae...
Primarily are caused too much light and/or for too long, combined with a high level of nutrients in the water.
Fix these and the algae will not survive.
Some ways to eradicate algae...
#s 1, 2, 3 & 4 are the best long term solutions to algae problems.
#5 can can work wonders under the right circumstances.
(1) Rub-a-dub dub scrub scrub scrub... Break out the scouring pad and get busy.
But, believe it or not, leave some decor covered with it, the algae on the object will be using the nutrients other algae would need to grow, your controlling more algae growth by giving it competition.
(2) Lowering amount of light.
Lights on for no more than 8hrs unless growing plants, preferably 6 hours.
Not everyday is bright and sunny, and some days are partially cloud, subdued/ambient lighting when not at home is just fine.
(3)Temp -77'f, algae does better at temps above 77'f.
(4)Nutrients in the water.
Reduce feedings. Increase PWC's (partial water changes).
Nutrients that algae need are nitrogen, phosphorus and potassium. These nutrients come from the waste the fish and beneficial bacteria, uneaten food, and the decay of dead plant/algae material. tap water can be high in phosphorus.
Phosphates, tap water can be high in phosphates and algae thrives on it, adding a phosphate removal media to a filter can help greatly... But, every water change puts more right back in. You could go to RO or Distilled water for water changes in this case.
(5) A UVS (Ultra Violet Sterilizer), kills the algae and some types of other free-floating unwanted biologicals. A algae bloom is still a possibilty with UVS if not set-up properly.
(6) Certain fish/shrimp/snail species are good for eating specific types of algae.
(7) A Diatom Filter, removes algae from the water.
Not a permanent solution.
(8.) Filtration using a 'Micron' pads will remove algae.
Not a permanent solution.
(9) A 'complete blackout' of the tank for at least three (3) days, sometimes longer will kill the algae bloom.
This option will not adversely affect your fish, but you can move them to a holding tank if you want to.
Not a permanent solution.
(10) Algaecides, use VERY carefully, preferably don't use'em at all as they can cause harm.
Not a permanent solution.
Goodluck and HTH.
