need answers

Hi i have a 75 gallon tank 20 gal sump 1 bursa trigger 2 three stripe damsels 1 tomato clownfish 1 firefish 3 snails 3 unknown crabs and a chocolate chip star fish in the tank live sand and live rock i have a fluval 406 and a aquaclear 70 filter how often should i clean the filters and how can i get rid of nitrates with out doin a water change the tank is about 6 months old