The way I cleaned my tanks that held stuff previously is with vinegar and water. What ever you do, don't use any ammonia based products because they will screw up your tank for a little while after being set up.
You can use a number of things to clean it. Even bleach, but just remember the health of your fish depends on you rinsing VERY well. How large is the tank? The bigger it is the harder it is to clean, and if you use something that could hurt the fish, the harder it will be to rinse it enough. So something like vinegar will help you, but not necessarily disinfect 100%. Though on thinking about it you probably don't have to worry about your fish catching anything from rats, and all you are wanting to do is clean out any urine, which the vinegar should do. :
Are you sure all the seals are holding? Often old tanks used for something else besides fish have problems with the sealant. Fill your tank somewhere you can check this without leaking all over your house, before you set it up in its permanent home. Best of Luck!
When I get a tank that has been used previously I put it outside in the yard and fill it with water and add bleach, then cover(so nothing can drink the water. I usually leave it over night then rinse it with the water hose very thoroughly. then refill with water and add enough dechlor for that amount of water and then add extra and allow to sit over night and then its ready to use
Your local fish store should have aquarium safe silicone sealant. When the aquarium is dry, trim off some of the ragged sealant, be careful not to cut it back to far. apply another coat, just thick enough to cover and go out over the old onto the glass. Wet your finger, or a teaspoon and smooth it out. If you want a really neat line, first mask with tape the outer line that you don't want the sealant going beyond. Then after you apply the sealant, just let it dry for 10 or 20 minutes, pull up the tape, and you will have a clean neat line of sealant. The sealant needs 24 - 48 hours to cure before filling with water
Fish in the Frozen North. 0 F this morning. If you are interested in the IDIDAROD SLED Dog race, the restart is happening this morning in Willow and you can see pictures, articles , updates, and videos all week long at http://www.KTUU.com Enjoy!