May fatal month for fish

Most fish had survived for several months, however May 2017 was a fatal month for the fish purchased as a large number of fish died . The list of the fish who died in May 2017 are
– Koi fish
– Guppy
– Tetra
– Goldfish
– Molly

The water for the fish was changed periodically, yet the fish died. The koi fish may have died because the sucker catfish was introduced in the bowl, however other fish died rather suddenly . The guppies are also sold out at the local aquarium, and there are no guppies available, only tetras.
Has anyone observed the same problem with their fish in May

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