Your tank may have a rubber or plastic boot, which allows the tank to stand (where appropriate) and creates some protection if it bumps into things. Check underneath the boot periodically for corrosion. To check for internal rust and corrosion, you need to have a professional visually inspect the inside at least once a year. To do this, the inspector drains the tank slowly and removes the valve. Using a special inspection light, the inspector checks for corrosion, cracks, debris and other possible damage. (This service must only be performed by a trained professional at a qualified service center - do not empty the tank yourself.) 

Once the tank passes the visual inspection, the inspector usually puts a sticker with the test date on the cylinder (they're not used in all areas). Professional Bali scuba dive facilities will not fill a tank without a current visual inspection sticker.Because tanks are also subject to metal fatigue, they must receive periodic pressure tests called hydrostatic tests. The test subjects the cylinder to very high pressure You also need to store your tanks properly. Keep them in a cool place, especially when full, because the pressure of compressed air rises when exposed to heat. 

Full scuba tanks left in a hot environment, for example, can rupture the valves' burst discs. Store tanks with between 10-20 bar/100-300 psi of air to keep moisture out. If you store a tank without using it longer than six months, have thecial testing tank, and evaluates how much it expands and contracts, which reveals metal fatigue or stress.When a tank passes the hydro-static test, signifying that it can safely hold air at its rated pressure, the tester stamps the test date onto it.

Professional Bali scuba dive facilities will not fill a tank lacking a current hydrostatic test date. Your instructor will tell you what local or national standards relate to your cylinder hydro-static testing. Standards vary from country to country for example in the United States and Canada, you need to have your tank hydrostatically tested every five years. In central Europe, steel tanks require hydrostatic testing every two years, and aluminum require it every five years.