You need to save money and start researching your fish envirnoments. There are only a couple dozen fish that will live in Level 1 environment for extended periods without additional help.

Using Vitamins is a temporary measure, but expensive.

In one game, I'm doing a bit a research to find which fish at least make it to adulthood in level 1, I have the extra tank and the aeration system, temperature regulator, cleaning snail, and the nutrition and curing plants and certain hybrids still decline in health.

So save for that tank, which will help you breed more fish and then save for the envirnoment research (or skip right to the research if you have some good breeders to use.)