I know someone who has a Juwel Trigon 350 tank (one of those corner/crossbow ones) and i'm helping them out with setting it up.
We're going to turn this into a Discus tank, we are either going to have 7 Adult Discus fish in there or 15 small ones (i know this sounds insane, but we will be giving them back to the lfs as they start growing and swapping them for smaller ones to put back in).
What i'm looking for is an external filter (Don't really rate the internal one) which will do a really good job for these Discus and will also keep the water crystal clear throughout the week , water changes will be done weekly of 40% (probably more), preferably i'd like the filter to be under £110 but if it's something exceptional i might be able to convince the owner to go a little further
Oh before i forget, the tank is fully cycled and the PH/KH of the water are on the required levels for the discus.
I don't have any personal experience with discus but from what I've read they require/appreciate more water changes of a larger volume. Somewhere around 3x a week, 50% of the volume. But that's just bits I've picked up here and there, I'd make sure one WC is enough. They're sensitive guys.
Anyways, by external to you mean canister? I'm not sure if I know of a canister that can filter that much water that's around your price range. You could check online classifieds to see if anyone's selling one. If you mean HOB or canister, definitely go with a canister. They filter the water much better and are pretty much dead silent in a room. I've had experience with both and I adore my canister. So much easier to clean and I love being able to choose the media I want to use, instead of using cartridges.
AquaClear HOBs are a bit different, as they have a basket that you can put whatever media you want into, and I have one. I still prefer my canister over it. It is one of the better HOBs out there though.
I'm a fan of Hagen's Aquaclear (now marketed as Fluval Aquaclear) HOB filters as unlike cartridge filters, we can decide the type and volume of the media to use. I have two AC70's on my 60g, one is a dedicated bio-filter and the other is just mechanical.
As for canisters, Eheim, Rena and Fluval are generally considered the top contenders. Although cost is always a consideration, "good tools aren't cheap and cheap tools aren't good"...Sometimes it pays big dividends to pay a little more to have a high quality filter that will run quietly and trouble free for years.
Some folks (not me) have made great finds on used equipment. Sadly fish keeping is all too often a short lived hobby and some folks will sell perfectly good setups together or in pieces. You might find something this way.
[thinking out loud]-> I also know that it is often said that Discus require extreme water changes. Still, I wonder if this is largely due to the simple truth that our aquarium filter systems just don't filter/purify the water well enough.
Oh, I would also recommend LOTS of live plants, as I do believe Discus have a pretty large bioload. Floating and fast growing stems are your best bet with helping to control ammonia/nitrate/nitrite. The fish will also appreciate the foliage.
As much maintenance as the person is going to have to with water changes alone, I'd caution to stay away from Aquaclears. With the bioload going on they are going to need a lot of maintenance.
I know when I finally got a canister for my 150 I went with: Rena Filstar XP 3. I have a second canister on this tank but that aside, this one is fairly easy to use and there are places online selling it for a reasonable price. Be wary you may have to buy the spray bar extra because what it comes with at the cheaper prices is going to potentially have too much flow for the discus.
You could probably do really good planting, frequent water changes and go with the Rena Filstar XP 2.