I am very interested to see how your tank goes. Currently we have a 125 with quite a few corals, (softies and zoas) and a mandarin. We have had up to 12 seahorses in ours with no problem. As long as you do your water changes, and are skimming adequately with a proper clean up crew, 20 should be no problem. Not to mention, if you are looking for seahorses, please let me know, as i currently have more than 50 available in beautiful colors. If you would like to see any pics, just let me know. Thanks.