Phosphate has several negative effects if present in a too high concentration. First of all undesirable algae growth might result. Making an aquarium an unpleasant sight.
Phosphate also blocks or retards the growth of corals and calcareous algae.
One of the scientific studies shows that a phosphate concentration higher than 0.05 mg/L decreases the growth of calcareous algae by 90%!
Washing fish food, which is a rich source for phosphates in a sieve after thawing reduces the input of phosphate tremendously. Also avoiding use of phosphate-rich low-grade activated carbon helps keep phosphate levels as low as possible.