Well, we can always try to explain the problem...
It's called osmotic pressure. Osmotic pressure is a measure of the amount of dissolved minerals present on one side of a membrane versus the dissolved minerals present on the other.
What happens is that water (just water, no minerals) will diffuse across the membrane to attempt to equalize the amount of dissolved minerals on each side.
Now, SW fish are in an environment with lots of dissolved minerals, a higher level than what is in their bodies. This means that water is constantly diffusing out of their cells into the ocean trying to even out the level of dissolved minerals (take away enough water, and the mineral concentration inside the fishes body would equal that of the ocean).
Now, this never actually happens, because the fishes body is constantly ingesting water to keep its internal supply up. If the fishes body didn't do this, its cells would shrivel as the water left them. result: death
also, keep in mind that the fishes body has to have some mechanism to remove excess salt from the water they ingest.
FW fish are in an environment where their body has more minerals than the surrounding water, so water is diffusing into the fishes body trying to even out the level of minerals. Without intervention, this would cause the fishes cells to swell and burst. Let's just say that's bad...
Now, the fishes body counteracts this by continually excreting water from its system.
Now, you say "convert a fish from SW to FW" It's simply not possible. the fish do not have the correct internal anatomy to carry out the functions needed to survive in the different environment.
That is the same as saying we'll convert your dog to living underwater by dunking him/her under water each day for progressively longer periods untill the dog adapted. Great, except the dog will never adapt. Its body can cope with the environment for a short period (holding its breath) but eventually, the stress will become too much, and the body will no longer be able to cope, and the dog drowns.
Basically, take a SW fish, and slowly lower the salinity, and the fishes cells would end up bursting (no, it's not like *splat* fish guts everywhere - you'd need a microscope). The SW fish has internal systems that conctantly take in water, and it never has to worry about excreting water. Put it in FW, and the water is constantly diffusing into the fishes body, and it doesn't have the proper mechanisms to get rid of it all.
There'd also likely be a problem with kidney function and cleansing waste from the blood...
Now, there are SOME (a VERY FEW) species of fish that can adapt to both environments. Usually these species native habitat is at the mouth of rivers where they may see large shifts in salinity on a regular basis, may even swim out to the ocean, or upriver a bit. BUT, these guys have evolved to live this way - their body has all the tools it needs to deal with both salt and fresh.
Of course, they probably don't want to work in one 'mode' exclusively all the time, and would be best in a brackish tank anyways...
As for your friend, what is the salinity in his tank? What is the hardness in his tap? add any supplements to the tank?
anyone know a bit more about the papio? how adaptive to changes in salinity are they?
: ...anyway if anyone can find anyinfo about this it would help me out a lot becuz im trying to make a bluegill live in saltwater fun lil experiment.