Have removed my internal filters from my Juwel's and replaced with the external Eheim 2026 (adjustable flow) in one and the Eheim 2324 (with thermofilter) in the other - both fantastic. Eheim are quite pricey, but well worth it - none of the constant filter maintenance, plus they're very quiet (had to keep checking that they're actually on when I first installed them!).
Kim