Phosphorus (P) is another key macronutrient.
Phosphorus is present in our aquariums in compound known as a phosphates (PO4). Phosphates are the food for aquatic plants. Ideal amount of phosphates in aquarium is from 0,2 ppm to 3 ppm. Phosphates (PO4) are made from organic waste such as waste from fish, old leaves of plants etc. Phosphates have to be in concentration higher than 0,2 ppm. Otherwise, your plants won’t thrive and there’ll be an algae in your tank.
If you have strong lights and not enough fish, you have to supply phosphates (PO4) artificially. You can use Monopotassium Phosphate (KH2PO4). Test kits are good to monitor phosphates (PO4) level in your water but it’s not necessary to use them.
You should add roughly 2 ppm of phosphates (PO4) in form of Monopotassium Phosphate (KH2PO4) weekly. You can make a solution of this dry salt and distilled water or you can just use teaspoons to dose.
25 gallon tank needs approximately 1/16 tsp of KH2PO4 3 x a week. It’s something about 2 ppm a week.
Here’s a tip:
If you have in your tank GSA (Green Spot Algae) – in the picture, there is a big possibility that your tank suffer from phosphorus (P) deficiency and you have to add some extra phosphates (PO4). GSA is a good phosphate deficiency indicator.