The electrolyte is the solution that allows electricity to flow from one plate to the other. There is a different electrolyte for different metals.
For copper, the electrolyte is copper sulfate. For zinc, the electrolyte is zinc sulfate. For steel, the electrolyte is ferrous sulfate. All three chemicals are readily available online; copper sulfate is often used as a root killer and can often be found in plumbing departments of the local Home Depots and Lowes, etc. However, it’s best to find a reputable company that can supply a clean a pure chemical, such as The Science Company, thus avoiding any unintended reactions due to impurities!
When preparing the solution, you really only need to make enough for what you need in the tank. Because the electrolytic process does not dilute the solution, but rather maintains the same strength, which allows the solution to be used for years.