how I can model the sides of this tank (the sides I marked by red)?
which tool I must use?
I try different tools but no good result.
Initially, you could use the top face of the cylinder, inset it a few times, turn on proportional editing and pull out the very centre poly, which would give you the bulge. Then use boolean to add the two sqaure sections.
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You could also try: Make a UV sphere or a round cube, flatten it to the proper shape, then chop off one half. See image.
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thanks.great help.
Glad it helped. 
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