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

Suckerfish kills koi fish

While purchasing and keeping fish it is important to know which fish can survive in the same tank, other many of the expensive fish will die or be killed fairly soon. The suckerfish keeps the tank clean, so it was put in a tank with a koi fish. However within 24 hours of being kept in the same fish bowl, the suckerfish killed the koi fish. The discoloured koi fish was found floating on top in the bowl
The koi fish had lived for nearly 6 months without any problem at all, though the water was becoming cloudy
The fish and the bowl was stinking due to the dead fish and the water had become extremely cloudy
A chocolate molly was kept in the same bowl, and survived for 24 hours, however to keep it safe it was transferred to another fish bowl.
In an aquarium, the suckerfish is surviving with a large number of fish, only in the fish bowl, some of the fish are dying quickly