Same tank as me - just looking at it now I can't imagine having 3 of those beasts in it. The original internal black box takes up enough room and I have an eheim external running as well.
Eheim 2215, though sometimes I wish I'd gone for the 2217. Having the second internal filter helps tho - if I didn't have it I would definitely have gone for the 2217.
Often find them second hand on ebay or the classifieds forum on here.