Welcome to the wonderful world of Mastiffs! I have a Dogue de Bordeaux or French Mastiff, female. She's about 29" at the withers and weighs about 90 lbs. Many of the Mastiff breeds are larger, few are any smaller.
If you will only be getting one dog, there are doggie seat belts you can get to help restrain the dog in the back seat of the car. However, they tend to be addictive and the next thing you know you've got one or two more.
Shelters are good places to get dogs, you could also consider rescues. Many times if the dog has a health issue the rescue organizations will have a good idea how much it could cost you on average or they may well have gotten health issues corrected by the time the pet is adopted. This is definately an issue you will want to explore if you have to watch your funds.
There are many good foods available at a reasonable price. I go through 80 lbs of food for 1 French Mastiff and 2 Rotties every month. I use Nutro for Large Breeds and it costs about $35.00 per 40# bag. Hope that helps. You want to be very careful about over feeding and make sure you are feeding a good quality food.
As far as your aquarium is concerned, obedience training and a good roomy crate will help you to avoid any accidents with the aquarium. My 3 are pretty good house dogs. I don't allow too much playing and rough housing in doors so it's not been an issue with us.
Please do keep us posted, let us know how the process is going and feel free to ask whatever questions you feel you need to. The only dumb question is one not asked.
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PHRottn
HRottn@PetHobbyist.com" target="_blank">My Email