I did a search but could not find it. What is the best way to clean the inside if a metal outside holding water tank Thanks in advance

What kind of "metal"? How old? How large? What needs to be cleaned (algae? crud? toxic waste?)

As said the type of tank and what needs to be done make a big difference in the attack plan. The round SS tanks are easy to clean with just a hose from top opening if you open the center drain in bottom. The square tanks are a lot harder and someone may have to enter and scoop out dirt. You can also have someone at top with broom to get dirt into the water and drain with a siphon hose. (drop garden hose to bottom of tank with some water - start suction to ground level and stir up the dirt so it is removed in the water).

I pressure wash mine every 6 months and throw some Dettol in for good measure.

I have plastic ones though.