Others have given good advice, so i needn't repeat. To the present, realize that the odd behaviours of the molly and guppy might well be due to problems from before with ammonia or nitrite. Fish sometimes live through cycling but have internal issues and die later. You don't want to be putting medications/treatments in a fish tank without good reason, as many of these are stressful in themselves and can make matters worse.
I would re-home the algae eaters, if these are the "common" ones like Chinese Algae Eater. Click on the shaded name to read about this fish, there is a photo too.
As someone wisely suggested, don't use pH adjusting preparations; and stop the salt. Some of your fish can tolerate salt, some cannot.