Get a phosphate test kit. Tap water is usually loaded with phates, that's why RO
water is worth it. I buy it at my LFS
for $.25 a gallon, so weekly water changes only cost $1.50 plus salt, which I buy in buckets... An acceptable number of phosphates is essentially "0". Anything over .03 ppm
is considered high, which is why doing things to prevent them are important. So get a test kit, and you can start by checking the water in your aquarium, and for giggles I would check your tap water. I'm interested to hear the results...