I am new to the hobby too.I have had two damsels and they both died in less than 5 days...I was devastated(just ask my hubby).I have a 10 gal
tank as well and just had one fish at a time.What killed them was over feeding and high ammonia and Nitrite levels(the under gravel filter was a factor too).This site has great info and I haven't given up I am doing more research and looking at getting a bigger tank.
One thing to do(please correct me if I am wrong) is when nitrite is high don't fed the fish until it comes down.If it isn't happening fast enough then try a 20% water change.
A rule of thumb for the number of fish in a tank is: you want to think about whatsize the fish will be when they are adults and then not more than 1-2 in per gallon.For example,.a Clown Goby is more suited for a 10 gal
because its max size is 1 1/2 in so no more than 9 or 10 Gobys in 10 gallon. tank.gallon.tanksAquarium Fish: Tropical Freshwater Fish and Saltwater Fish for Home Aquariums
(liveaquaria.com) has great info on what fish are suited for different size tanks.
Good luck!We can do this!