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30L is heading close to pico territory (or is already into it by some people's standards), which is the most difficult you can get in terms of keeping things stable, although a 30L simple hermit or mantis tank is a very different and simpler beast than a 30L reef. So...what all inverts are you wanting to keep? It's easier to pick critters first and figure out equipment based on that than to try to go the other way.
Seriously, and with all due respect, it is impossible to keep any type of marine tank if you are "not too fussed with it". They are by definition very high maintainance. Even if your goal was a tank just filled with SW and one mushroom you would need to keep your water parameters suitable for that life. If you do decide to go marine, fish are more forgiving than coral, both time and equipment-wise.