OK, ya, your checking correctly. Water to ground. Getting a few volts to ground, even 25 is not bad. There will be no amperage there. its the amperage that hurts. Also, since the fish are never touching the water and ground, there should be no harm to them. they will experience the 0V like when you put both leads in the water. Alot of times at my work, I'll get a reading of like 90+volts on a wire that is in fact de-energized. Touch the wire, nothing. Most Digital meters have such high internal resistance that they do not draw any current. this can in fact lead to erroneous readings alot of times.
but still, By all means, a GFCI is the best insurance. If there is an electrical problem, say a heater case breaks and now the live wire is in contact with the water. Everything will be good untill you touch the water and something grounded, then OUCH. GFCI will trip before you feel it.
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