Tropical fish sinking after water change

Question
As a gift to my roommate and I, my girlfriend recently purchased 5 tropic fish and a tank/habitat for them to live in.  The purchase was done on the recommendation of the pet store employees and the fish were fine for approximately two and a half weeks.  Each time the fish were re-introduced into their tank (first at my girlfriend's and next in our dorm room), they adjusted quite well.  

However, we recently decided that the tank was in need of a good, thorough cleaning.  Using no soaps, we did our best to clean the gravel, clean the filter, etc. We filled the tank with room temperature tap water and restored it to the condition it was in when we received it.

Then, something went wrong.  We fed the fish in their recently refreshed water and they were doing fine.  Then, without warning, they started sinking one by one.  One sank, would thrash about and swim upwards only to sink back down to the gravel at the bottom of the tank.  While some of the fish managed to continue swimming longer than others, eventually, all five the fish were on the bottom, each exhibiting the same behavior:  Skimming the surface of the water, eating some food, then sinking to the bottom of the tank.  One of the fish even attempted to leap out of the water several times before sinking in the same seizure-like manner that we observed in the fish who attempted to shoot back towards the surface.

I fear that we have killed our pets!  Sometimes, they move but mostly they stay stationary (but upright) on the bottom.  According to my girlfriend, the fish behaved in a similar manner after first being introduced to the tank.  She claims that after about two days they began to move around once again.

Answer
Matthew,                                                     Before I can answer this question I need to know a couple of things. How big is the tank? Do you have a heater? How did you clean it? How long did you have it before you cleaned it?Please send me this info so we can figuire out what is going on. TY, Tina