Incorrect concrete pad level where water tank sits

Hi all, just discovered the concrete pad level where the water tank sits is about 210mm lower than what was specified in the hydraulics plan. The water tank seems working fine. Builder is happy to redo the pad to the correct level if I want. Their hydraulic engineer said the lowered level doesn’t affect storm water. I don’t particularly want to change it if it has no effects. Meanwhile, the builder wants me to sign something to say I accept the wrong level. My questions are:
1. In hydraulic engineers original specification, he said the level MUST BE xxx. Now the level is quite significantly different from the MUST BE level and he is saying it is fine. Would it be really fine?
2. What other consequences can you think of due to this changed level? It may only become apparent later. I can’t think of any right now. But then I am no engineer.

Thanking you.

The only thing I can think of is that if the line is charged out then there would be less head if the rwt outlet is the hp. If the engineer says its okay, then it's okay.

I would need to see the hydraulic plan to comment fully but a lower tank level aids the inflow hydrostatics if it is a charged system which I assume it is due to their being a hydraulic plan. A charged RWT system only relates to the hydraulic head anyway which factors the top of the tank...not the bottom.

For the hydraulic engineer to say "must be" can only relate to the stormwater but this is irrelevant as the overflow pipe will self charge plus the stormwater system still needs to be laid and sloped irrespective of there being a tank.

But if the same size tank was used and base lowered, then consequently top of tank & HH is lowered

There is more HH when relating to a charged RWT system (from gutter to top of tank or to the water level if plumbed with a Supadiverta system). This is different to a (possible but unlikely) charged stormwater system.

The "must be" notation is strange if there was no reason for it.