This is Oz based but concrete is world wide.
We have an outfit called Mr Crystal which really knows their concrete tanks etc. A small bucket is expensive but well worth the how to booklet IMO.
Then you buy Xypex Concentrate which does the same thing at a lower price - that is used by pros to stop surface cracking on big slab pours.
Both you paint on outside and it wicks into the crack and seals things up.
Following the Mr Crystal advice on getting rid of movement (as long as the reo is intact) we've taken a 10000 gallon tank from bogging a cow right beside it to minor dribbles
Your local concrete expert will likely be able to translate