does anybody know of somthing that will clean my gas water heater

Assuming you mean decalcifiaction then I bought a 5 litre container of it from Cifec in Amposta. You just detach from your mains pour it in the pipe til its coming through and then plug it up and leave it for a bit. Rinse it out later, reconnect and away you go.
Bruixot

Hi Spanish Catfishing

If you mean you have a build up of carbon on your heat exchanger or if the flames are going orange or yellow on the burner then there is a simple but time consuming cure you need to strip the boiler  down and jet wash it with a pressure washer, try to work in the opposite direction to the heat flow and in the combustion chamber blow through from the exit jets to the gas entry point you will find that the gas jets push dust and cat/dog hairs into the combustion chambers and they get blocked, and eventually they stop working properly and then the heat exchanger gets clogged with carbon, it pays to do it at least once a year, remember carbon monoxide kills and if you have yellow or orange flames then things are getting blocked and the gasses may be escaping into a room

I pressure washed my gas boiler recently and boy what a difference it made and the boiler is now more efficient as well

Hope this helps Stuart