I am most familiar with Prime as that's what I use, but I think it either is sodium thiosulfate or is very similar to it. It's my understanding that the nature of the risk is that there's a list of stuff in the water it will react with and it has very good affinity with chlorines, chloramines, ammonia, nitrite, etc. so as long as there is some nasties around to react with it will, but if there's way too much Prime it will run out of it's preferred targets and move down the list, eventually reacting with oxygen. This can completely deoxygenate the water, killing livestock. (I'm pulling this from memory, but I trust someone will correct me if I'm misstating something.) Prime is supposed to have a safety factor built in where you can do as much as 5x the recommended dose and still be ok (i.e. not suffocate your fish), but obviously it depends on how oxygenated your water is to begin with and how much "stuff" you need to take care of. I don't remember if that 5x threshold only applies if you are treating the new water as opposed to the entire tank.
I have never heard of any problems when people are in the ballpark of the directions, including treating the entire tank. I don't think they are too hard to follow, especially if you have proper measuring equipment (I have small tanks, so I use a graduated medicine dropper), and am generally more afraid of forgetting to add Prime than adding too much.
I have also seen people kill fish with Prime. If you do your math way wrong or screw up measuring it can be deadly. But I don't feel like that is a particular mistake I am likely to make, so I'm not worried about it, especially when every alternative solution is a giant hassle.
I don't know anything about aloe in water conditioners, but man, it doesn't sound like something fish need. Aloe grows in arid areas, so there's no obviously ecological connection to freshwater fish writ large... Like, what is the pitch there?
