With small cases it is always a tradeoff between equipment and drive bays
I am not sure if there is space there or not, but have you thought about mounting the heatercore on the side of the case (sorta behind the 3.5" bays)? If it would fit, you could duct a fan to blow through and then probably still fit the eheim 1250 where the lowest 3.5" cage is. That would mean you'd get all the 5.25s and the top 3.5" bays, and the loop would be pretty neat and out of the way.