bubble build-up

We have about a 2" layer of crushed coral/live sand mix in our 75gal tank. We have been having problems with exccesive bubble build up in this base. All water tests have are good. Our regular 20% water changes have been every 3 - 4 weeks.
WE have the following in our tank:

50 blue legged hermit crabs
3 turbo snails
1 yellow tailed damsel
1 juvenile emperor
1 regal tang
3 ocellaris clowns
3 bubble anemone
green frog spawn
xenia
star polyps
button polyps
1 peppermint shrimp
1 cleaner shrimp
2 egg urchins

The bubbles also collect on the live rock, and sides of tank. Maybe a combination of things or one main culprit. Appreciate any thoughts.