I am thinking of using one Hydro Pro #5 sponge filter in a 40 gallon breeder. Is this filter too big for that tank? The filter is rated for 125 gallon aquarium.
Should be fine, they are recommended for 40 gallon up to 125 gallon as long as you don't mind the space it takes up (4.5" H x 5.25" W plus another 5" H for lift tube ). You could use pairs of smaller ones too if you want. I believe all of the other parts of the filter for #1-5 are all the same, just the sponges are bigger. A lot depends on what you use to power it as well. A good air pump rated for your tank size or a power head is an option. If you are using the tank to actually breed, probably the air pump. Their filters work really well and are pretty cheap. I use a #2 in my 29 gal currently along with a HOB. I would also suggest using either an airstone or one of their diffusers inside the strainer to make the bubble much finer.
The "pro" is their articulated coarse foam. The non-pro is their fine foam.
The "Pro" versions are made for higher flow rates and extended service intervals.
I use ATI's products and like how the parts interchange.
You will be surprised how much muck they trap.
The standard ones are actually more effective than the pro versions for lower flow. It might also might be better for a breeding set up to have a finer sponge? I don't know much about breeding though, so I could be wrong. If you plan on using a strong powerhead to drive it, definitely the pro.