If a DSB
will become a sink, then a shallower sandbed will do the same just faster. The theory goes that phosphates and other materials accumulate in substrate until full and then leech back into the system. The only answer to this would be going bare-bottomed.
I do not believe this to be the case in a well maintained DSB
, but it is a debate you should definitely research and decide which is the best for you. All three systems mentioned can be equally successful in my opinion.
If you do the DSB
in the refugium, you can change it out if needed. However, critics using the "sink" theory claim that a refugium actually collects the bad stuff quicker and is useless. CC
could also be used, but you would have to vacuum it frequently. To my knowledge, heavy metals, phosphates, etc. would still become bound in the substrate.