Bio Bag in Hangover Filter

How frequently do I need to change the bag of little bio rocks in my hang over filter? It also has a carbon bag inside it as well. It's been awhile since I changed it. I didn't want to have the ammonia spike, but right now my nitrates are spiking. I have a 30 gallon tank, and in the last two days I changed out 20 gallons of fresh RO water from my aquarium store. My ammonia and nitrite levels are zero, but the nitrates are sky high. I thought maybe my nitrate tester was off, so I tested a sample of the RO water from the store, and the result was zero nitrate. What do I do? Could it have to do with the bio bag in the filter not being changed? I was just shocked that the nitrate level didn't decrease with the 20 gallons of new RO water. Any suggestions would be helpful. Also, any good ideas for filtering the sand when I do the water changes?

 

something dead in your tank? anywhere else nitrates can be besides your bag? need more info on tank.

 

I did have a fish die about 2 months ago. Never found him, even when I changed tanks. I had a 29 gallon hex tank that I hated. So I recently moved to a 30 gallon regular tank. I kept the same sand and live rock, didn't add anything new to the tank. It has about 3 inches of sand on the bottom. I also have about 26-27lb of live rock. I have a hippo, a damsel, a six line wrasse, a pair of clownfish, a coral banded shrimp, a hermit crab and a few snails and an anemone. I have a 50 gallon hang over filter with a 50 gallon protien skimmer, and two power heads creating a good flow.