How remote key of popular car brand working wide range

I did a job at 434MHz that used tiny 0.25mW transmitters and a tiny PIC. Battery life was good - over a year because the transmitter consumed virtually nothing for about 1 minute then woke up, transmitted its small payload and went back to sleep (about 50 ms up-time). There were about a hundred transmitters all monitoring the temperature of individual freezers - basically they were "alarms" to warn of impending defrost should one of the freezers become faulty. One central receiver.

Conceptually I don't see a difference - you have a bunch of transmitters all of which can randomly talk to a central receiver. Transmitter current consumption when transmitting was about 30mA (from memory - it was in the early 90s so my memory is not that great on this).

I got the transmitter modules from a UK company called Radiometrix - they were FM 434MHz devices.

Because of the potential for transmit collisions I would also recommend a display that all customers can see that tells them their request is being processed.

Changing the protocol will not help ! The water tank system is disturbing the 433 MHz band no matter what protocol you use. The protocol only determines HOW the data is transferred on the channel you're using. You still use the same channel so both systems will still disturb each other.

A solution could be to have your device on a different channel in the 433 MHz band. Here is a list of 433 MHz channels. Try to find out what channel each device uses, maybe they use the same channel maybe not. If they use the same channel maybe you can use a different channel for your device. You probably would have to buy a different transmitter then. Not sure how to determine what channel that transmitter will use. Maybe all cheap transmitters use the same channel.

Another solution will be to use a different frequency band. If the use of the 315 MHz band is allowed in your country (you must check !) you could get a receiver and transmitter at 315 MHz. This will be insensitive to 433 MHz so it will not be disturbed by the other 433 MHz signal.