Yes, it is normal to see some foaming when using API MELAFIX fish remedy, and it is not harmful as long as the proper doses were given. In terms of duration, the foaming of this product can last a variable amount of time depending on what is dissolved in the water. Water that contains a lot of dissolved organics or proteins can experience more foaming that lasts for long periods of time, but in general, if foaming lasts more than a week after discontinuing the treatment, we would recommend more frequent water changes and the addition of ACTIVATED CARBON or BIOCHEM ZORB™ filtration media in order to eliminate any remaining fish remedy. Make sure that skimmers are turned off during treatment, as this would contribute to excessive foaming. It may be prudent to turn the skimmer pump flow down when it is restarted after treatment, to account for fish remedy residues until they clear from the system.