Hey... I have a problem that I have a fluid in the tank but I can't see It... I try to break tank and add block again but thats doesnt help can you help me ?

Not really sure I don't really remember all the bugs 1.12 and earlier had, though it could also be some rendering mod like optifine is screwing with it not sure. Might have a bit better luck seeing if anyone on our discord who still actively plays 1.12 remembers an issue like this. (Closing this as we are not actively developing 1.12, so even if there is a bug on our end it isn't going to get fixed)