If your water treatment plant uses Chloramine, that could be the cause, or even chlorine from a high residual.
Dripping threw a carbon filled tube, or also having a metered dechlorinator drip may solve the problem.
This is why I never considered the drip method as reliable for me, I found it easier to to a large water change every other day, and treat the new water with a measured amount of sodium thiosulfate.
I also had very cold tap water in Milwaukee (as low as mid 30sF in winter) so I used a combination of hot and cold.
If the combination is used, it is advisable to drain the water heater occasionally (from the tap at the bottom of the heater) until the water is no longer slimy.
Minerals and other compounds can precipitate to the bottom of water heaters from the constant heating a cooling, and then become part of the water used in changes.
I would drain my water heater of about a gallon or two every couple months. and the water from the bottom is at times almost viscous if not done enough.
It is even part of the normal maintenance procedures in the manufacturers instructions (the material no one ever reads) because the mineral soup build up, can lessen the efficiency of the heater, if left to long just for general use.
Another problem with very cold water (it is often super saturated with gases) and can cause gas bubble embolism in fish if not allowed to gas off. An air stone placed directly under the drip, or the drip threw the carbon tube, can help with this.