I have a very big fishtank about 72 gallons or so. It is saltwater but my brother tries but can not keep the fish alive long enough. So I was thinking about getting a hamster. Could it live in glass fish tank? And are the hard to take care of and need a lot of time because I also have a dog.

Clean it out really good first and use a good thick tight mesh wire cover on the top. One that it can't stick its head into or chew threw. You might need to fix it so that the top doesn't come off easily either.

If it is 72 gallons that is a huge tank. The poor little hamster is going to get lost in it. I would get a smaller tank for a hamster - say 15 gallons tops. Hamsters are not hard to take care of. Just remember to feed them and give them water.

Have your brother start keeping fresh water fish instead and keep the 72 gallon tank. Salt water fish are VERY hard to keep. I had 21 aquariums in my home at one time years ago and I never lost any fish as they were all fresh water. I had a 50 gallon tank and had huge fish that lived a long time in it.

Omg! Yes you totally can! Just buy a screen top (so your hamster can breathe) and buy all the right things. Clean it REALLY well. As for taking care of your hamster, it is not hard, but it does take devotion. You can't decide you want a hamster and then neglect it. The dog should be fine with the hamster (just curious maybe) Make sure your hamster has fresh water and food
At all times and change cage once every week or once every 2 weeks. May I also suggest a dwarf hamster?