I wouldn't use these. It has been years since I think I heard of CO2 tablets but they never became common. I can't see them doing anything to be honest.
Plants need CO2 during the light period (daylight to the plants, when the tank light is on). And they need CO2 continually during the light period, or photosynthesis will slow and may cease. There is normally sufficient CO2 in an established tank to provide for the plants in a low-tech or natural set-up. The CO2 occurs from the breakdown of organics by bacteria, primarily in the substrate, and it slowly builds during the night. When the light comes on, the plants begins to take up the CO2.
Plants will photosynthesize to the max according to the available level of all 17 nutrients and if light is sufficient intensity. As soon as any factor in this equation is reduced, photosynthesis will slow. So I cannot see CO2 from dissolved tablets providing what is needed. I suggest you save your money.
As for someway being detrimental to fish, I am not certain, but I would be concerned if this release of CO2 is rapid and at night. The natural process I mentioned above is gradual, and unless the tank is heavily stocked the fish are not harmed.
Byron.