Hikari Bacto-Surge Foam Filter, XL (125 Gallon)

Q: does it come with stones, and can you use the hose that I already use for my air bubbler?

It comes with stones. In terms of the hose you already have, what's the size? It should be okay. Just for your information this doesn't clean 125 gallon fish tank. It doesn't pick up fish poops.

Q: Does this come in smaller size for 55 gallon tank?

The Bacto Surge Sponge Filter comes in sizes from a 5-gallon tank all the way to a 125-gallon tank. The 4"x4.5" for up to 75 gallons would be perfect for your 55-gallon tank. The UPC is 042055960441 hopefully this will help you find it!

Q: Would several of these be effective in a 300 gallon stock tank? I have 2 large koi.

I keep three in my 135 gal. I rotate the cleaning of them so that there is always a good amount of beneficial bacteria. I also have a large Pro 3 Eheim on the same tank. Sponge filters are excellent mechanical filters as well.

Q: whats the best air pump to use for two of these?

Any pump will work. If you have 2 and only a single line airpump you will need a splitter or y connector of some kind. Or you can use a control valve that lets you set different pressure for each line. Personally I have a pump with 2 outlets. It runs 2 separate tanks. You probably want a pretty good sized pump for 2 of these however.