From what I have heard, fish food should be vac'ed out in 5 minutes, except bottom feeder pellets. Fresh/frozen veggies can be in the tank for 4 hours, or overnight to remove snails. I would assume they break down much slower as they are not dehydrated and processed to increase shelf life. Research your fish though, not just plecos and gouramis like veggies, a lot of fish will eat them.