Don't take this the wrong way, but just because your tank is doing fine now does not mean it will be fine in the future. 20 fish for that small a tank is way to much. The pet stores do it because they sale the fish fast and they never really stay in the tank that long.
You say it's fine and a lot of people hate on you, but that's only because they have experience and have done the same thing you have done now. I started a freshwater 29 gallon tank and threw in 4 dwarf gourami, 1 golden gourami, 1 blue gourami, 2 clown loaches, 1 Rainbow fish, 2 mollies, 2 sward-tails, a lobster (Took him back, trying to eat everything), 6 neon glow fish, and 1 Pleco. That's 20 fish in a 29 gallon tank that I just put up THAT DAY. :-/
I was a real noob, and my tank was fine for a while. I had it up for 2 months and kept using the "Tetra Safe Start". Well than it happened, all my fish started dieing and I couldn't figure out why. My tank was starting to really cycle the Safe Start couldn't handle it anymore. Plus my golden gourami was terrorizing everything. I went from 19 fish down to 10 fish. Than when everything was said and done, I had only 6 fish left. I was upset and mad. I took my fish back for no credit and put all my fish tank stuff away. I than found this website and started researching. I found out all I did wrong, and learned from the mistake and set up a 29 gallon salt water tank. It's been 3 months and no problems. I only got 3 fish that belong in a 29 gallon, and one fish that is pushing it.
I am planning a 180 gallon tank right now. It won't be until a year from now, but every fish I have can live in a 29 gallon all their lives but one. The Blue Spotted Puffer according live aquaria needs a 50 gallon tank for it's whole life, yet I have researched and talk to many experienced hobbyist in my local area that kept them in a 29 with no problems. My fish has been happy swimming around for 3 months. I plan on no more fish until my 180 gallon is set up and my 4 fish I have now will be moved to the 180.
My tank is as follows.
1 Flam Hawk
1 Six Line Wrasse
1 Clown Fish
1 Blue Spotted Toby (Puffer)
That's it, and I even think that is pushing it!! I got a BIG skimmer for the tank a BH-1000 reef octopus. I got 50 pounds of live rock and 60 pounds of live sand. I have a canister filter that I clean every week to make sure the Nitrates are kept in check, but it keeps my water clear and clean!
The point is, 20 fish in that 55 gallon is not a good idea. You got fish that need a 125 gallon Minimal!! Live Aquaria suggest 180 gallons for a yellow tang (I think that is going overboard myself, but I do think they need at least 125).
To the new guy. Remember one good saying! A understocked tank is easier to keep up than an overstocked tank. I also feel that it looks better as you can really appreciate the fish you have in the tank. When you have too much there is information overload!! For a 55 gallon tank I would suggest doing your research and not listen to the LFS
. They will sale you anything. I have found one in my area that is REAL good about the fish you buy, they asked me before I bought my fish what tank I have. They refuse to sale a fish that will not have enough space or a fish that will go in a tank that is not cycled. I am so glad I found that store, but they are few and far between, so you got to be careful of where you get your advice.
Do not get the Trigger are any Tang that needs a big tank to swim. I talked to one lady that has a yellow tang in her 29 gallon and she can't figure out why it keeps bumping into the wall hard. Some fish have it bad, don't make the mistake of giving your fish a bad place to live for the sake of looks. With a 55 there is a lot of cool fish you can get that will live their whole lives in that size of a tank. When you get a tank set up and perfect it's worth it. Good Luck!