DUDE MY PUFFER JUST DIED!!!!

I'm soo sorry to hear that your puffer passed. More than likely it was something to do with the water, if you just did a change did you add a dechlorinator (prime) to your water BEFORE adding fresh to the tank? When you get the test kit I'd test the water just to give you closure. Check for salinity, Ammonia, Nitrate, Nitrites. Black belly/grey belly is your typical sign that something is really wrong as you already found out. Could possibly be a parasite of some sort? But I'm betting something with the water since puffer was fine prior to the change.
Oops. almost forgot, are you using marine salt or just aquarium salt that can be found in Wal-mart? FYI. in case you weren't already aware--Puffers are actually saltwater fish and require the marine salt.

 

I'm sorry you lost him.

Make sure you get a marine test kit and not a freshwater kit, they are different.

 

Personally I think saltwater is more difficult than freshwater, some may disagree. There is definitely more that goes into the tank to keep a stable saltwater environment and it's not as forgiving as freshwater.

 

This is a 20 gallon, right ?

 

If you keep it as a saltwater tank you should increase the salinity slightly. After you bought about 20 pounds of live rock you could get a few. There are a lot of gobies that will work for a 20 gallon tank. You could also do a single Ocellaris Clownfish.

 

The lowest price for live rock that I've seen in stores is about 6 dollars a pound, so you're talking about $120.00 for the rock.

With freshwater there are a lot more options. Guppies, a betta, some tetras, goldfish and many more.

 

Way way too big. You're really not going to be able to do cichlids unless you did shell dwellers and a select few dwarf species.

 

flowerhorns would be too big in my opinion---they can max out at up to 18" I've read, but most of them hit about 11"...IF I were you, I'd avoid the saltwater in a 20g...that's fairly small and like Anthony said--very unforgiving when it comes to the slightest change. Saltwater needs constant monitoring to make sure your tank is stable, and definately needs to be tested with a marine test kit. If your changing that 20g from salt to freshwater I'd rinse it with freshwater like crazy--and honestly I'd probably do the 5:1, let it sit the 10-15...rinse it and let it sit with the rinsewater 5-10...then rinse it again and let it air dry for 24 hours. For freshwater the list can go on and on..depends on what your wanting-->species only, assortment-community, planted..blah blah blah.

 

You're going to have to completely empty the tank or drain as much water as possible, refill it, drain it and refill it again several times to rinse all the salt out. The same goes for the filter.

 

Just stir the sand up with your hand each time you add freshwater, unless you want to remove it all and throw it away which I doubt you want to do.