Marine Considerations

[andyG44]

I am reading here and there about how to make a marine aquarium. One quick question: why do I need live rock and even (live) sandy gravel - whereas in the tropical tank I do not need any rock at all ?
 
I have read and discussed with people that have experimented that the gravel, ornaments and filter trays/foams/media harbour bacteria that perform the ammonia conversion. But why do we need the addition of rocks in the marine tank? Why isn't the gravel / filter enough?

 

[DrRob]

Simple answer is that you don't. There's nothing to stop you running a tank with a classical external. The general issue is that corals and marine inverts tend to be more intolerant of nitrates, which build up with normal external filters, the Berlin method with live rock gives a degree of anaerobic filtration which will clear the nitrates.
You could, in theory, run an external with  denitrifying filter. If the expense of live rock is the issue then you may not end up saving much if you go down the tech route this way.
 
The rock also acts as a buffer for the pH and dissolved calcium levels, but the substrate will do that.
 
Finally it offers an ecosystem opportunity, by introducing a mixture of critters that you get with the reed aquarium.
 
I've read repeated questions about the use of live sand, many reckoning that it's simply a gimmick, live rock though always comes with good reports.

 

[Donya]

why do I need live rock and even (live) sandy gravel

 
You can do a bare-bottom marine tank. But, if you want substrate, you should use pretty fine-grained sand. Aside from pH buffering from increased surface area, another reason for sandy substrates instead of coarser options in marine systems is that coarse substrates easily turn into a bristleworm farm. Bristleworms are almost a guarantee with a marine tank; you can end up with them even without live rock. While the worms beneficial scavengers in low concentrations, coarse substrates like crushed coral or large freshwater-style gravel allow extreme population booms because they trap fine debris much more rapidly, even when vacuumed regularly. When the substrate gets overpopulated with worms, it can also become full of their shed spines, which is a nasty painful mess for any soft animal that touches it. I unfortunately have saw that in one of my early marine tanks and it's very hard to get back under control once it happens. So, unless you have a very specific need for coarser pebbly substrate for some reason...go with fine-grained sand or go bare-bottom. And, unless you have some specific need for silica sand, go with aragonite - it buffers the pH and is also less likely to scratch the tank. Like Rob said it doesn't have to be live sand; a lot of live sand will experience a fairly substantial cycling period anyway.