Hello: Hard or very conductive water will cause the anode to work too hard. This produces gas which usually is most evident on first use. A test would be to remove the anode and put a plug in place for only a few days to see if the gassing stops. If so, a less active rod or a powered anode can help. Another trick which works only for a short time is to put an automatic vent in the line. Trouble is, in hard water they clog up pretty fast 😕
ps. A Marathon heater, (which needs no anode) is another way to get it done.