Hey,
Not to rain on anyones parade here, but let's step back and consider some things about the eventaul size and habits of the fish. The two Clowns you have will geventually grow to be 4 - 5" in length and 15 gallons is pretty small for that size. Fish swim laterally the majority of the time and 15 gallons is a pretty short run. Adding more than two fish to a 15 gallon is not very fair to the animals IMHO and is very risky without sufficient biological and mechanical filtration including a skimmer. Too little water to have sufficient water quality safety buffer if there is any kind of an imbalance in water parameters.
I have a 20 gallon long with two small Damsels, a blue and a Blue Green Chromis. I have 35 lbs of live rock, 4" of live sand, and a skimmer. I can't see putting a third fish in there as there is just not that much swim room, even though I would love to.
Fire Fish gobies are very skiddish and more prone to jump out of smaller tanks any time you do tank maintenance. Other gobies who have more of a den territory are not at such risk to jump and generally require less tank area.
The reasons above are a great part of why liveaquaria recommends nothing smaller than a 30 gallon. I know a 20 gallon is a stretch, but the 30" of lateral swim room lets me get by. Minis and nanos, less than 30 gallons, are very much limited by the fish they can truly support long-term.
Food for thought.
