I'm late to the party, but I wanted to make one last general case for sumps. I know you,
@Gaz111 , have experience and know much of this. But, because others are reading it, I thought this might be useful broadly.
Just to be clear, I am not trying to start an argument with anyone out there, but for me, I prefer sumps. In my experience anything that is a bonus for a can, a sump has but more so:
Lots of media possible -> sump more so
Added water volume -> sump more so
Can hide equipment -> sump more so
Both can run silently. Both have some level of leak risk (really any time you decide you are going to remove water from a tank for processing - even HOBs to some extent). What I have found is that can filters can be great options for folks who want an off-the-shelf solution for filtration. So, new aquarists, or those worried about the DIY aspect of sumps. And there is nothing wrong with that! The caution I have is that cans often end up as a set-it-and-forget-it type of system. Out of sight, out of mind. That's understandable but not optimal.
But there are some areas where sumps are miles ahead of cans. For a start, accessibility for consistent maintenance is one. A sump gives you access to everything. A well designed sump is so easy to maintain! My best sumps take me under 5 minutes a week to maintain.
Another bonus is the greater ease of redundancy. For things like returns, you can run two smaller pumps if you like. If one goes out, you aren't falling off a cliff. Same for heaters, which are notoriously problematic these days. Cans just can't approach that level of insurance and peace of mind. Plus, you can selectively put equipment on battery back up for much less investment than if you had to run and entire canister off a battery.
The last thing I will say is that sumps really are flexible as a platform. I have a tank that in the last decade has gone from a planted tank, to a cichlid tank, and finally to a
marine tank. Over all these changes, I never had to substantially modify the structure of my filtration! Need a skimmer because it's a reef tank now? No problem. Dropped it in! Want to explore what socks do or don't do for your system? Put them in, take them out. Your choice. Sure, I added a wall for a refugium at one point, but I did that wile the system was going.
It's just that sumps like that aren't an off-the-shelf solution. That, in general, a sump is not. And I fully understand OP's concerns about drilling a tank. Having done it many times now (all of my tanks are drilled even if they are running air driven filtration as I have automatic water change set up on them all) it no longer concerns me. But that first time was anxiety inducing, as was the firs time I drilled a larger tank (who wants to ruin an expensive tank?!!!). But, for those who either have a pre-drilled tank, can find someone who can/will drill a tank, or are capable themselves, for my money, a sump is simply a better platform for filtration.
