If you really want to knock it down, there are a few steps you need to take prior to doing so just to insure it comes back. Like any medical task, there isn't 100% return on it.
Arapaima are odd when it comes to reaction to MS 222 (Tricaine Methanesulfonate). Due to the fact that they are air breathers, it becomes one step more complex. John's article shows a great example of how to move one using it.
With this situation, you'll want to pull the animal into a different pool to administer the drug, then have a third tank (with water from the main tank) for it to recover in prior to reintroducing it in with other animals. I say this because if you try to recove him from the MS222 in a tank with others, there is the chance that they will kill him while he's knocked down.
Put water fromthe main tank into the two additional tanks. Make sure both are in areas with plenty of room to work around them, and both are low enough that you can work in them to support the animal once he goes down.
Move the fish into tank #2. Make sure he doesn't freak out too much. Slowly add the MS222 (email me for the amount to dose amount based on animals size and water volume:
mheyde@minnesotaaquarium.com). Once the drug is in the water, it will take time to react. Add the drug in sections...not all at once. The first hit will be larger, then have addition premeasured amounts in case the first batch doesn't take. When they go down, they go quicky. Be ready to support the animal in your hands in an upright fashion. Once he's down, you'll be able to examine the fish closely. Feel the abdominal walls for the rocks. The only thing I would suggest at this point (due to size of the animal) is try to slant him nose down and gently massage the gut. Hopefully any loose rock will dislodge and fall out. If this doesn't work, don't push the issue. You are more likely to do more damage by forcing them out backwards rather than allowing him to pass them on his own.
To recover him, move him directly into tank #3 with clean tank water in it. Hold him in a "normal" swimming position. Move water over his gills using your hand or a small power head. Not too much, but enough to get good gill contact time with non-medicated water. It may take awhile for them to recover. It's a little un-nerving the first time you see your fish knocked down and not coming around....but they do come back. I've take some fish deeper than I ever intended to, and with time, they came back. Let him recover for a day in the isolated tank prior to putting him back in the tank with the other fish.
Do we for sure know it's rock? Could the bumps be something else? Do you have pictures of it?
Link to the thread about John moving Sam:
http://www.monsterfishkeepers.com/forums/showthread.php?t=49048
