Fairly big gap at floor around pipes of hot water tank. For mice prevention I read some where you could fill gap with aluminum foil then seal. Is this correct. If so what kind of sealant should be used? Need help right away. Not sure but I think they might be chewing the insulation. Right now I’m using cotton balls soaked in Peppermint Oil, undiluted, in small glass containers (3) around the hot water tank. I did find 1 mouse that came through a gap around an electrical outlet. Had Peppermint Oil placed on floor under the outlet. The mouse died within 15 minutes. Haven’t seen any more but that’s not to say there may be more. What can I use to seal around the electrical outlet. I’ve got it covered with duct tape now. I’m elderly and disabled and live alone with my dog. I’ve been doing minor repairs myself but these 2 situations I have no idea what to do other than keep using Peppermint Oil which, by the way, I have sitting all around the house in strategic places such as doors, windows, under beds and furniture, behind fridge washer dryer stove, cabinets drawers, shelves high and low, all corners and especially under kitchen and bathroom plumbing. I’m a little paranoid about ants and other creepy crawlers as well as mice.
I think your best bet would be to get a can of spray foam sealant. It expands to tightly seal gaps. It's very easy to use and you can get it just about everywhere. It won't smell as good as your peppermint oil but will probably do a better, more permanent job. Good luck.
Peppermint oil is not as effective against mice as traps and poison. It should go without saying, but keep your dog away from the oil, traps and poison - whatever you end up using.
The first step against pest infestations is to remove the attraction. Keep your rooms clean and all food in sealed containers.
You can seal and insulate gaps and cracks with expanding foam as JACK suggested, but keep in mind this stuff is not for exposed exterior use, and should not be used inside outlet boxes. Drywall and siding/foundation repair are better permanent solutions, but are more expensive and require more work or expertise. Duct tape and aluminum foil (or aluminum tape, for that matter) will not effectively seal cracks where insects and mice can get in.
To deal with mice already inside, you can use traps that kill mice such as glue or snap traps, poison pellets, or humane traps that lure mice inside a small box so you can relocate them. If you use humane traps you must release the mice very far from houses so they don't become a recurring problem to you or your neighbors. Be sure to check the traps regularly and keep pets away.
Crushed aluminum foil is not a good packing around hot water pipes or even cold water pipes. It is soft and conducts heat very well.
Much better is to lay galvanized steel mesh ("hardware cloth") or sheet flashing around but NOT touching the pipe.