how much food is over feeding?

[goldie]

the tank has been running for almost 8 months. I started it with 2/3 dry rock and 1/3 live rock. I have good flow, the 2 stock pumps and an extra one positioned much lower down. I am using the stock skimmer, I wouldn't know how to change that out? It does pull out some disgusting green slime. I only am testing salinity,temp,nitrates,phosphates and calcium. My water changes are usually about 4 or 5 gallons, I guess I should start doing larger ones. Is it ok to scrub the rocks with a toothbrush while they are still in the tank. I usually try to do this just before a water change