Okay, I would stop and get some API test kits tomorrow. The directions come with them and it's very easy to do (take sample, drop liquid in, wait for it to turn color, match to color chart)
Taking the plants out might have dropped your pH way too quick, gars can be sensitive to abrupt shifts from what I recall... also a really low pH could have killed off your beneficial bacteria, triggering a recycle in your tank without you realizing it... also adding tap water might have pushed the pH back up, again a drastic shift, the stress could have killed your gars. If your tank is in some sort of recycle then you might be dealing with high ammonia -- but the other fish should have died off before the gar if this were the case.
Just some ideas... really impossible to tell until you get the water tested. Your other fish could still be in danger.
Agree with Richard though, the three floridas dying off sounds like some sort of external poison or something.