It can be so frustrating. You’ve just cleaned the tank a few days ago, and you already see brown spots forming in the tank. Algae, or diatoms, are the “grass of the sea,” because like natural grasses on land, algae grow readily anywhere and everywhere they can find a spot.
Algae can harm the fish by blocking needed light and by taking space. If the algae get too out of control, the dead and dying diatoms can create toxic levels in the tank and cut down on the oxygen your fish need to live.
If you see a buildup of algae on the glass or tank decorations, clean them with a sponge, soft cloth, or toothbrush. Let the broken up algae settle to the bottom for a while. Then finish by filtering out the gravel with a water vacuum.
If, however, you are tired of this incessant algae battle and want to prevent the buildup from happening, there are steps you can take to reduce the growth of algae. One is to use the aquarium light only when you are viewing the fish.
Lights increase algae growth at a surprising rate. So, as long as you have enough lighting for your fish to get around in, from outside the tank, keep the tank light off, until you want to admire the tank for awhile.
Use limestone sand or gravel instead of silica sand, because the silicic acid feeds the algae. Silicic acid is also in the water, so treating the water can help. Finally, filter new water for your tank through specialized filters designed to remove the acid, before you put it in the tank.