Well, part of the reason I avoid quarantining is because I feel in my tank, it's a more natural enviroment. There are rocks to hide behind, currents to be felt, algae to be eaten. Maybe some people do set up a quarantine tank like a natural habitat, but wait, how's that possible? If that's the case, why don't we set all of our main tanks like QT to avoid disease? I do firmly believe stress is the main cause of death and disease. I feel if you give them as natural of a surrounding as possible, there's less chance of stress. Here's my steps in getting a new fish.
1) Go to the store, find a healthy specimen, and ask them to feed it. I want to see it eat.
2) Take it home, put in a 1/8" tubing which drips my tank water into the bag, I use the drip acclimation method (i'll be adding 1 drop of Lugol during this process soon).
3) Once the lights are out (they're usually out though because my lights turn on at 3pm) I introduce the fish into the tank, bag water and all *gasp* :lol:
At this point, the fish will usualy swim behind the rocks and I'll see it poke it's head out once in a while, and nibble here and there. I'll introduce some frozen food since that stimulates eating best and they usually eat.
You can find Lugol's at your LFS, it's made by Kent and it's $19 per bottle, but it'll last a very long time. I've even made 2 mercy purchases, fish that were sick, but I thought they had a shot at recovering. The reason I don't do it anymore is because it wrecks havoc on my water parameters due to the heavy feeding, but it's possible.
There are also other factors you have to consider, like if the fish was collected w/ Cyanide. If they were, it's hard to tell, and they'll die no matter what if that's the case. Also, I try to avoid new fish, but since you state the hippos have been there for over a month, I'd say it's safe to rule out cyanide collection. Another guage I like to use is the "Eagerness guage". When the fish has been in captivity for a while, the recognize humans as the bringer of food, and when they seem eager versus shy, that's a good fish to get.