I just bought an old used 30 or so gallon aquarium form the local second hand store for only $5. only problem is it has a build up of calcium around the top as well as I would need to reseal the seems.. I mean it holds water fine.. already tested but I rather be safe then sorry so I’ll reinforce them at the vary least.
So My big question is what it the best way to clean and old used tank to make it look ‘new’ again.. IS there some kind of solution you can make to help clean it? Or what?
Anyway as always thank you for the help I just can’t wait to get this tank up and going.. that is after I buy everything else except the hood it came with it.. I just have to test and clean that too...

I'd bleach the tank, scrub all the icky stuff as best as I can, rinse rinse rinse rinse, dry it out in the sand, and then reseal the seams.

The easiest and quickest way to clean off those hard water deposits is with muriatic acid. It will clean that tank up immediately and make it look bran new without any effort.

If you choose to use this method, mix the muriatic acid with water to make a 1 part acid 3 parts water mix. When you mix it add the acid to the water, you NEVER want to add the water to the acid. Wear rubber gloves and work in a well ventilated area.

Muriatic is completely safe to use to clean the tank. It is completely neutralized by tap water. You can find muriatic acid at most hardware stores and at any pool supply store

My used tank looked the same way. I layed it on it's side and poured viniger inside it and on the top and let sit for 15 to 20 minutes. Then I layed it on another side and did it again until all the inside and outsides were done, scrubbing after each soaking. After that I used baking soda which seemed to take off what the vinigar didn't and I heard that it neutralized the vinigar. Anyway that worked for me and was cheap. Good luck with whatever method you use.