If the tank is clean then it is not necessary to re-sterilise it before fermentation. The fermentation process itself is one of the strongest sterilising methods there is.

SO₂ solution has a slight sterilising effect but doesn't kill everything, you can use it before racking a wine from one tank to another.

If you happen to acquire a second-hand tank in a dirty condition then ideally steam it (steam should flow from the outlets for at least 30 minutes) using a steam generator, then pressure wash and scrub until it is clean. Steam is the most thorough and safe method of sterilising, that's why it is used to sterilise the bottling line before filling.
The alternative is to use chemicals such as peracetic acid or proprietary cleaner/steriliser.
Almost all tanks are AISI304 grade stainless steel, any cleaner/steriliser must be chlorine-free.

New tanks usually have a protective plastic adhesive-film covering the outer surface and the rim inside. This should be peeled off carefully. The inside of the tank should then be scrubbed out with a scrubbing brush and cold water.

With flat-bottom tanks: at the end of racking/bottling when you tip the tank over, with hose connected and valve open, to drain the last of the contents, the tank needs to be raised above the floor. I always keep a flat bottom tank on a wood pallet, which raises it by about 125 mm, which is just enough.

Using a wood pallet allows you to move the full tank around with a small hand-operated pallet truck.

If you have dished-bottom tanks on legs (or a stand) then a pallet is not needed. These are easier to wash out with a high-pressure washer; I use a hot-water high-pressure washer.  To collect the 'washings' I use a low 27 litre bowl, placed under the valve outlet, then tip it away down a drain (which must go on to the land, not to the public sewer as acids destroy the digester).

A flat bottom tank can be washed in a similar way but needs to be laid down on its side. Then turn it upside down when leaving it to drain and dry.

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