It sounds like your aquarium water is too hard
How to be sure
GH – 14 °dH / KH – 10-15 °dH
HARD – Generally acceptable for most community aquarium fish. Perfect for alkaliphiles (alkaline loving fish) like African lake cichlids.
GH – 28+ °dH / KH – 20+ °dH
VERY HARD – This water is too hard for most fish.
Water hardness is the measure of the total dissolved salt content in water or the quantity of certain metallic ions. Namely, calcium and magnesium although calcium is the most significant. These ions are generally present as either hydroxides, carbonates or bicarbonates. The general hardness (GH) refers to the total content of these combinations of salts and can be divided into carbonate hardness (KH), which can be removed through boiling the water, and permanent hardness, which remains after boiling.
Step 1 – Treat your aquarium
Perform several small water changes with reverse osmosis or rain water to bring the hardness down. If RO water is not available, using boiled tap water is also an option.