If you are thinking of bigger things, you will need to scrap the idea of an internal as it simply won't cut the mustord
Fluval make the best internal filters on the market, but also the worst exturnal IMHO Avoid the Fluval 05 series completely unless you an pick one up FOC. Nowerdays, you could't pay me to take on off your hands, after my old one gave me an electric shock. After a few years they can get niosy and the flow rate tends to drop, if they make it that long without making you wash the carpet enexpectedly. (They have a habbit of leaking from various places just outside of warrently) The final nail in the coffin for me is the fact that they are a pain to work on, needing re-priming every start and also need weekly attention due to cloggs.
Eheim are considered the best, but if you are on a budget, they might not be for you. The classics are cheap, but can be harder to maintain than most others. The Eccos are as bad as Fluvals. The Pro range is where you want to go realy with Eheim, preferably Pro2, as these are proven and easy to use. They start at about £100 for a small one though....
Tetratec is what I run exclusively on my freshwater tank now, and I can't fault them. The EX1200 can be deliverd to your door ready to go for about £70, but keep that seal well Vasalined. If you don't apply Vasaline at every cleaning, you will need to replace the seal after about 2-3 years as they dry out without it. This is the only fault as such I know of in the Tetratecs ATM, but they have bearly been arround for 4 years.... Most other issues (this one included IMO as you are told to apply vasaline to the seals in the instruction book, but of caurse who read them
) are caursed by the users themselves.
Aqua one filters I have no direct experience with, but I have read may good reiews on them, and the few that use them on here seam to like them...
Cascade filters have high by-pass apparently, so avoid like the plague
Thats my run-down on exturnal filters for you
for more user reviews, check out the review thread in the Hardware and DIY sub-forum
HTH
Rabbut
Edit to add, how could I forget my old Rena.... Good clean workhorse, that does what it says on the tin, after changing the media configuration. The supplied media in these filters isn't any good and needs replacing realy. Also, keep main seals in stock, as they dry out quickly. Once dried out the seal needs prompt replacement or the filter will start to leak
