Before I reply I just want to say that I am debating this in the friendliest possible way. I know that on the net, because you can't see facial expressions or tone of voice that sometimes it's very easy to get the wrong impression in a reply. I have no malice or know-it-all attitude, simply debating a subject!
Okay....
so, the whole nitrates to nitrogen gas thing. There is (from what I've read) tremendous debate on the effectiveness of LR doing this. The first thing is that not all rock is created equal. From my own personal observations, tonga branching and marshall island rock are not very porous at all. Many of the pieces that I have seen up close have no pin holes, etc.... just basically a rough surface and that's it. Alot of the aquacultured rock is just riddled with holes of various sizes so there is quite a difference when it comes to the 'type' of rock. Then there is the question of the rock being a mechanical filter. No question that it does, but like any filter it can become clogged. Again, that will depend on so many factors, cleanup crew, biological load within the tank, and the rocks themselves. The rock is a finite material, it can only hold so much. If the water, actually the nitrates in the water cannot reach the anaerobic bacteria deep within the rock, then the nitrates will not be processed. Which is why every so often, it's a very good idea to blast your rocks out with a powerhead or turkey baster and get some of the built up detrius out of the tank (another reason to have a filter, when you blast your rocks, the filter will grab up some of the gunk and you can remove it from your tank when you clean your filter!
) Live rock often has coralline algae growing on it which can block more of the pores, and the lastly, LR is well known for holding onto phosphates which can fuel unwanted algae growths.
Before anyone says "you must hate LR" let me say that it is a good thing to have in your tank. I wouldn't consider having a reef tank without it! If you can afford buying the rock then go for it. I am just saying that in a fish only tank, it isn't a necessity like it is in a reef tank.
The last comment I have is that in the US at least, you can buy a HOB filter for $30....LR around here goes for around $5 to $8 a pound depending on the rock. Yes, you still have to buy decor....in my own tank I think I spent around $50 (some things I had laying around from other tanks). So in my situation if I had spent the money for the decor and the filter on some rock I would have ended up with 10 to 18 pounds of rock. Which means I would
still have to buy some decor. The plus side to having fake decor is that on occasion you can take them all out and clean/bleach them. No build up of debris and no coralline growing all over everything. If the fish get sick, no need to remove them to another tank either, just dose the main tank as you won't have to worry about meds binding with the LR.
Again, if you were to do a reef tank I'd say you must have LR but in a fish only situation then no you don't have to have it. It's great if you can afford to do it but you can keep fish quite well without it.
JMO, IMO, etc...