I'm using a Marina Slim S20 filter on my 20gL. The outflow isn't too turbulent and the intake tube has a sponge prefilter on it so that would help prevent any fry from getting sucked up. Plus, the intake can be dropped lower into the water so it would work if there wasn't much water in the tank. Although, the outflow would then become far more turbulent as it has to cascade down further.
Sadly, unattractive as they are, a sponge filter probably is the best bet for that big of a tank. AngelsPlus has some custom ones that fit nicely into a corner and if you get the 40ppi, you don't have to worry about any fry getting sucked into the filter.
Sponge Filters for aquariums
Another option is using a gang valve and hooking an airpump up to several smaller box filters like these:
Lees Large Triple Flow Filter
Using two or three of the large size should provide enough filtration for a full spawn tank.