Oh heavens no! The blue stuff in the cups at a lot of pet stores is a form of antibacterial agent to protect the fish from infections and to help keep them healthy. Someone has seen that and thought that it was dye. Some people put a darkening agent called Atison's Betta Spa that contains the essence of Wild Almond Leaf extract in it but the advantages of this are mainly to neutralize ammonia and to protect fish in uncycled and unfiltered tanks. It can calm your fish if it is ill to have some it in the water but otherwise I do not see a total need for it. A lot of owners buy leaves from dealers in Thailand to float in the tanks of their bettas but I have never done this. My betta boys always went a little crazy when their water went dark, yellowy brownish and did not like it. So I left them in the clear clean water they liked.
Blue dye will do nothing but kill your pet. So do not listen to everyone and their advice. The medication they use in the stores looks blue because of the methylene blue in the ingredients but it is not necessary and it is a cop out that pet stores use to keep from having to do a lot of cup maintenance on the bettas. They think it will make them live longer with no care.
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