Cleaning out an old fish tank... Is it safe to use chemicals?

Don't use any soap, detergent or cleaning chemicals. Use water and an aquarium glass scrubber (get one from any fish store) to get dirt off. If the tank has lime deposits (whitish or other-colored chalky stuff stuck to the glass) that won't come off with just water and scrubbing, use white vinegar to soften up the lime (let vinegar-soaked cloths lie on the glass for a while if the lime is really rock-hard). You can also scrape gunk off with razor blades, but there's a risk of scratching the glass if you use a blade.

Once all the crud has been scrubbed off the glass, rinse the inside at least five or six times with water.

If you have used vinegar, it would also be a good idea to test the pH once you've filled the tank with water (the potential problem with vinegar is that if any residue is left it can acidify the water (i.e., cause low pH). If the pH is lower than that of your tap water, rinse the tank a few more times and test again.

You can use a glass cleaner (like Windex, for example) on the outside glass; just don't let any of it get on the inside of the tank.

It's true that you can use bleach to sterilize a tank, but it's not generally necessary, and it can be hard to get all the bleach out afterwards. Even after you'ved rinsed it 10 times, it still smells like bleach, which means there's still bleach in there somewhere.

If the tank has been dry for a long time (probably true if someone dug it out of their garage or basement), it's quite unlikely that any fish pathogens have survived.

Fish Tank Cleaner Chemical

You can use a 10 parts water to 1 part bleach solution. soak for up to 15 minutes , rinse it extremely well, let it dry for 48 hours.

This will kill the most bacteria and it is perfectly safe.

make sure the tank still holds water before adding fish.

other things to clean tanks or the decorations with are,

white vinigar, non iodized salt, hot water, lemon juice.

You can use bleach, like noseless said. If you need to "scrub" the tank, I personally use a bit of aquarium salt and warm water and gently scrub at the lime or dirt deposits with the salt.

Yes as the others said you can use bleach because it dilutes so well, but you really should rinse thoroughly, and let it dry in the sun. Dirt can beremoved with sand or a razor blade.