Hi lenenkie, i have a 2 ft tank as well and i am having a sand bed or around 2.5 inches deep.
Before i first started, i had this question as well. But most of the information i found online says that it actually takes about 6 inches of sand bed to make it a DSB and its takes a ling long time to cultivate the bacteria that consumes nirates to turn it into nitrogen? If i rem correctly that is..
I am a newbie as well, i find that weekly 15% wc and a oversized skimmer plus polyp lab biospheres is doing the job well for me at the moment. Plus im overfeeding almost everyday.
Nirates have been at 10ppm for the past 3 days.
Just my humble feedback