According to the American Veterinary Medical Society freezing aquarium fish is inhumane, unless the fish is previously anesthetized, clove oil isn't proven to be quick or painless and CO2 is accecptable.
I'm not positive, but I think putting a fish into a small container of water and then dropping several Alka-Seltzer tablets into it would be utilitizing the CO2 method.
The AVMA's guidelines on euthanasia:
Amphibians, Fish, and Reptiles
Clove oilBecause adequate and appropriate clinical trials have not been performed on fish to evaluate its effects, use of clove oil is not acceptable.
CarbondioxideAmphibians, reptiles, and fish may be euthanatized with CO2. Loss of consciousness develops rapidly, but exposure times required for euthanasia are prolonged. This technique is more effective in active species and those with less tendency to hold their breath.
CoolingIt has been suggested that, when using physical methods of euthanasia in ectothermic species, cooling to 4 C will decrease metabolism and facilitate handling, but there is no evidence that whole body cooling reduces pain or is clinically efficacious. Local cooling in frogs does reduce nociception, and this may be partly opioid mediated. Immobilization of reptiles by cooling is considered inappropriate and inhumane even if combined with other physical or chemical methods of euthanasia. Snakes and turtles, immobilized by cooling, have been killed by subsequent freezing. This method is not recommended. Formation of ice crystals on the skin and in tissues of an animal may cause pain or distress. Quick freezing of deeply anesthetized animals is acceptable.