I think each system has its place and is useful in certain types of setup. It's up to the hobbyist to understand the pros and cons and plan something that will work for him/her. It's my opinion that closed loops are more worth it in nano/small/medium tanks, but not so much in bigger tanks.
The main pros & cons of a CL are, IMO:
Pro
- Looks: no powerheads visible, and inlets/outlets can be well hidden
Con
- Someone mentioned cost effectiveness, but as far as I've heard, that's the main con: Major electricity usage.
- As someone mentioned, you have to get the flow right. That means properly sized & engineered overflow and drains, as well as return lines. Otherwise you get a lot of noise, as well as stir up your sandbed, get microbubbles, etc.
- Any moving mechanical equipment like an Oceans motions needs regular maintenance, and gets clogged every so often, or in some setups are no end of trouble.
- plumbing failures, leaks (probably minor if you plan well)
If I were to set up a large tank (say 200g+), I would go with hidden circulation powerheads--Tunze, Koralia, Vortech, etc. I've seen it artfully done so you see very little or nothing. They can also be cleverly hidden inside a center column or back wall of custom rockwork. But you have to be able to pull them out for maintenance. The hassle would be mainly in the setup. If you could place them in a way that they're hardly seen or completely invisible, this would be best.
In smaller tanks, it's hard or impossible to hide the relatively huge Koralia's or Tunze's, so a closed loop starts looking much more attractive. It would be my choice. And while electricity usage may be relatively greater, practically it's often quite ignorable for a small tank (say 75g-).
And of course, there are endless possible combinations of closed loops and a sump & return, or with in-tank powerheads. If there's one thing we learn in this hobby, it's that there are many ways to skin a cat successfully.
These would be the major factors in my choice:
- Can I make it look good? This means keeping all equipment as invisible as possible.
- Can I make it run silent? We love a silent tank.
- Will it create the flow my tank needs?
- Can I pull it out/take it apart for maintenance as easily as possible? We dislike maintenance, but we hate hassle and complications even more.
- Is electricity usage acceptable to me? Your pocketbook and priorities will have to answer that one.
- Does it have a good track record of durability, minimum maintenance, minimum breakdowns, etc.?
You can research all of those online. There are ingenious ways to solve all problems, come up with by very smart or just plain persistent people.
