Well shoot. Hate to say it but if you're in love with the juice pre-fish taste, you may have to try a different delivery device to see if it remains inconsistent.
I've heard that the wicking system on the nova tanks can leave some of the flavor behind.. as if some of the juice ingredients are able to wick better than the rest of the formula, so you're not wicking 100% of the juice.
Also the dry hits on the nova have a pretty unpleasant plasticy sort of flavor. Maybe that off flavor combined with the juice your using winds up tasting "fishy" How unfortunate
When the novas are "on" they are quite sensational. When they aren't wicking well it can get kinda gross. I think your situation is to the extreme.