The FX5 has as many known issues as the Tetratec's. One
There was a dodgy batch of FX5's with motor units dropping like flies due to impeller failures and electronics control failures. As Bitteraspects say's, just do a google search
Now, I'm not narrow minded enough to think that the FX5 is rubbish due to a few hundred hits about one common problem specific to one batch of filters. Both the FX5 and the Tetratec are seemingly good filters for reliability. Check out the Tetratec issues in detail, you find that it is the
EX700 ONLY in one batch ONLY 6 months ago thats has had issues with leaks. With the FX5, it was
one batch, 12 months ago with motor unit issues. Now on one above members analysis, keeping comparisons fair and un-bias, both are cr@p filters due to this. If we come back to the real world, we realise, once all is said and done, both will be good, solid, reliable filters if you avoid the batches that were known to be dodgy, and that are still sat as stock in certain retailer's warehouses
Now, you are down to four filters here, as the Aquaclears are not available in the UK (easily) and the Tetratec's aren't available in the US (easily). So, you have the Rena XP3 to look at, an Eheim 2080, a Fluval FX5, and in the UK, Tetratec EX1200's and in the US Aquaclear's.
Someone asked how heavily stocked my tank is, to see what load it is dealing with. It's stocked to 1.5" per gallon on that rule, but they are large messy fish getting 4 heavy feeds a day, so the load is equivalent to something more like 500-600 Neon Tetras on normal feeds
All the best
Rabbut