I can't speak to the pimafix, because I have never used it. But I can speak to water quality issues: the causes, problems that result, and the ways to fix it! Lots of experince there.
You mentioned that you had problems with water quality, but believe that you have it fixed. Please do not be offended if I ask even more questions... sometimes people new to the hobby have recieved information from less-than-accurate sources, and need more (correct) info to really get to the bottom of the problem.
Do you know about the ammonia-nitrite-nitrate cycle in a new aquarium? This is a VERY important chunk of info. If your fish were in the tank thru the cycle, that completely explains the fins...and the death of the other fish. A tank that small is too small for even one goldfish, even if the tank had been completely cycled first (without fish). If the fish were in there thru the cycle, the ammonia levels must have been thru the roof...hence the burned look to the fins. He may have sustained even worse damage to his gills...which may kill him before the tail does.
Additionally, the filters that come with kits are notoriously under-powered for the tank size that they come with. Kits are sold to be inexpensive, and the best place to cut back on price in a kit is the filter. BAD move on the kit makers part.
All of which is my long winded way of saying that you need to get a MUCH bigger tank, and a better filter, if you have any hope of keeping fish...especially goldfish. Goldfish produce more waste than other fish of the same size...more waste means more ammonia. (I have 6 small-medium fancy goldfish in a 75 gallon tank...with a MASSIVE Eheim cannister filter. I am pushing the limits pretty hard, but the tank has been cycled for 8 years and the water tests perfectly weekly.)
What might have saved this fish (and the other one) is MUCH larger and more frequent water changes. A tank that small would have needed DAILY WCs, 50% or more each time, minimum, to get thru a cycle with fish in the tank. I am glad to see that you are getting the water tested, but really look into getting a drop-type test kit of your own. A cycling tank needs to be tested daily. A test for pH is also needed. Ammonia is more toxic at some pH levels than others.
Did you throw out the old filter pad and put in a new one? Much of your beneficial bacteria for the cycle is in your filter material. Do not throw it out, just rinse and squish it in water removed during a water change to remove excess crud, and put it back. Change it only when it starts to fall apart. If you threw it out, you basically are starting the cycle over again, although there might be some good bacteria in your gravel by now.
Something else to know...not sure where you are getting your info but if it is from your local fish store...get very suspicious. Sales people do NOT always know what they are talking about. Get your info here...no one here is looking to sell you anything, and DO know what they are talking about...most of the time!
