About six months ago, I purchased a 3.5-gallon tank with 3 guppies and one aquatic snail. Almost immediately, one guppy died and had to be replaced for schooling reasons. Since then, we've had no problems and were even able to introduce a new aquatic snail. Sadly, two days ago one of the guppies died. We have no idea why, but we think he may have been hit by a knocked-over decoration. We replaced him with a fancy guppy that looked a little bit different than the rest... he had a blue tail and the others have had red (I'm not sure if this actually matters). We saw him interacting with the other guppies well, but the next day he had died. The other guppies look fine and healthy so I'm not sure what went wrong. Maybe the fish already had a disease from the store or maybe the guppies gave him a disease they had but were immune to? I just want to make sure that our current, and future, guppies will be safe.