I did quite a bit of reading before i put my DSB
in and I feel like it really did help to reduce my nitrates. Full disclosure I only had the tank for a little over a year. I just had to take it down to move so i can't speak to the long term of a DSB
From everything i have read it can only cause a crash if you disturb it. If you want burrowing fish or inverts i would probably put 3-4 inches in and that will be good enough for most of them. Because they constantly turn up the sand you will not see the nitrate reducing effects of a DSB
. They are keeping the sand oxygenated and so the bacteria that consumes nitrates will not grow.
If you want a DSB
i would strongly suggest donig a remote bed. I kept mine in the sump (approx a 2' x 2' x 7") for my 90 and it was great since i was feeding so heavy because of my anthias. This also allows you to cut it off from the system if you ever need to disturb the bed. I believe some people suggest changing out about 1/4 of the bed every 2-3 years. You would need to make sure none of the water gets in the display when you do that though.