This is a tough one, because you haven't been given a lot of time. Most likely, the bug bombs will be using some combination of pyrethrins or pyrhethroids, which are some of the most common insecticides around. Unfortunately, these are also toxic to fish, so you're right to take action to protect them.
Ideally, your landlord would have given you a bit more warning about this. If nothing else, you have a right to know what's being sprayed around your apartment, in case you have any allergies, asthma, or other conditions that might be sensitive to the insecticides. Also, if this fogger is their only plan of attack against cockroaches, don't expect it to solve the problem for very long. But this is a digression.
The plus side is that these do break down in the environment after a day or two, primarily I believe from exposure to ultraviolet light. So if I were in your shoes, I think I would do something like this:
- Move the fish, the food, and any supplies that come in contact with the tank water (nets, buckets, etc.) out of the apartment. Keep them there for at least two days, preferably three or four. Really, I'd move anything fish-related that time and space allows.
- On a 55, I assume you have a pretty hefty biofilter, probably a canister? Move that out of the apartment as well, and hook it up to the temporary tank where you're housing your fish. This will help keep their water quality in check, since it's a smaller system, but it'll also keep your biofilter healthy and free from contamination. (I don't think the pesticide would hurt the bacteria, but it's one less thing to worry about.) Do this even if the holding tank already has an established biofilter -- nitrifying bacteria need to eat too.
- I'd turn off any internal pumps or airstones you might still have in the tank, and fill it as high as possible. I think the fogger works by small aerosolized droplets of insecticide that come in contact with the infesting bugs, but I'd still try to minimize gas exchange while the chemicals and fumes are in the air. I don't think it makes sense to drain the tank first, since you'd increase the surface area potentially exposed to pesticides.
- Cover the tank tightly with saran wrap, so there are no air gaps. That clingy stuff would probably be good to use. Then cover the whole tank with a water-tight tarp -- plastic, not waterproof canvas or anything porous -- that's long enough to reach the floor with a decent 'skirt' around the base. Flatten the skirt and weigh it down all around if you can. I'm picturing sandbags, but whatever's practical on such short notice.
- Unplug the heater, lights, etc. The only thing I'd leave running would be the biofilter, if you had to leave it on the tank.
In the meantime, remember your fish are in a very crowded holding tank. Feed them very lightly every other day, and if you weren't able to move the biofilter with them, pay very close attention to the water quality.
At this point, anything else that might ever go in the tank should be either under the tarp, or ideally, out of the apartment with your fish.
Once it's safe for you to return to your apartment:
- Leave the tank covered until the apartment has completely aired out. I honestly have no idea how much of the insecticide will stay in the air after a treatment. If possible, try to get a fan blowing air from outside the building directly onto the tank.
- After removing the tarp, do a massive water change -- 100% if possible.
- According to Wikipedia, pyrethrins are broken down by strong acids, so you I would wipe down the tank glass and lid with white vinegar, and then with clean water. Some mild dish soap would probably work as well, but be extremely careful not to get any in the tank. If you have any fish tools you think may have been exposed, I'd soak them in white vinegar as well.
- If you have another filter available, load it up with activated carbon, which should be pretty effective at removing trace organic pesticides from the water. I recently saw some small relatively cheap internal filters at my local pet chain that would work well for this, if you don't have anything else on hand.
- Let it run like this for a day or two is possible. It wouldn't hurt to do another 50% water change after a day or two, but it might not be necessary either.
At this point, I think you're ready to move the fish and biofilter back in. I hate to say it, but if you have one or two fish you're not quite as emotionally attached to, I'd move that one back in first and see how it does. If it still looks healthy after a day, move the the rest in.
I've never gone through this process before, so these steps are just what sounds good to me. It might be overkill, it might not be nearly enough -- I really don't know from experience. It sounds like a lot of hassle and risk, but I think it's doable. Good luck!