Ilford HP5 Plus Black and White Negative Film (35mm Roll Film, 36 Exposures)

Well you have to take the film out in complete darkness and get it into into your film developer tank. Once the film is developed you can of course have it in your red light lit darkroom, but I'm pretty sure you can't take the undeveloped film out in red, or any other, light.

This is b&w film that uses b&w chemistry only. No color chemistry should be used.

That is one roll of film.

I would agree that Dennis A. gave a great answer. The one additional bit that I would add is that back in the film days I used to carry a dark bag in the car. It is a double black bag with arm holes and I used that to advance the film for reuse of the roll.

Yes.

Most public aquariums are quite dark in most areas. HP5+ has excellent abilities to push process the film which allows you to shoot at ISO ratings up around 1600 ISO without introducing excessive grain. Assuming you are not shooting with a flash, 1600 ISO should give you the ability to shoot inside most aquarium's moderately lit areas. Just remember to tell your processing lab what speed you shot the film at so they can push it properly for you or if you are processing the film yourself, be sure to check the guidelines on the Massive Development Chart for HP5+. Areas that are usually much darker like where they usually have the jellyfish lit with special colored lights are still going to be the areas you would be better off shooting with a DSLR or mirrorless with better low light performance than film will give you.

Although not very accurate you can rewind, carefully and reload the same roll. For example, if 10 photos are remaining to finish the roll and the marker is at 26, you must release the advance mechanism and rewind the film, taking care of not pull the tip of the film into the cartridge. Just pay attention of the moment the film is released from the collection reel, in addition, the film becomes harder to curl when the tip reaches the cartridge, because of the fold made when the film was loaded on the reel of the camera. When you want to reload the film, try to fit the pickup spool end to the already folded part and advance photo by photo, taking care to cover the objective avoiding double exposure. Just in case, I do recommend jumping one more frame before exposing, in this case the marker should be at 27. From this point ahead, the camera operation is normal. Simple like that, no magic.

You should call the B&H customer service for that. As this is something that always changes depending on the batch.