Hi Peanuts
We have just re-done our freshwater tank and have plumbed two fluval 404s through the bottom. I was dubious at first but partner Mark (Pies) insisted ! It all seemed very easy, although Mark is quite knowledgable with his plumbing because of having the marine tank.
I am very pleased we did it this way, it looks heaps tidier, very easy to get the filter going, and we can have the tank right back against the wall. I can't think of any reason NOT to do it this way. Mark (with Suphews help) even drilled the 4 holes themselves !
You are welcome to come for a looksie anytime if you can't wait for the next 'installment' of the tank crawl )
Jane