- An excess amount of sunlight or artificial light encourages algae growth. Algae growing on fish tank gravel appears as spots. The spots can be various colors, including purple, green, gray, brown or black, depending upon the type of algae present. Adjusting the duration and strength of fish tank lighting helps prevent too much algae, as does introducing an algae-eating fish to the tank.
- Green algae sometimes causes spots to appear on fish tank gravel. While certain amounts of the algae is normal, it can be difficult to remove if it forms on the gravel. The hard, stubborn spots occur more often in fish tanks with low phosphate levels in the water.
- Black brush algae, or BBA, is actually a form of red algae that commonly occurs in fish tanks with low levels of CO2 in the water, low pH levels, and insufficient amounts of minerals and carbonates. It leaves grayish-green, black or purple spots on gravel that can be extremely difficult to remove.
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