Bottom Paint is formulated to apply via brush or rolller. Under these normal application methods, there is no need to thin bottom paint (reduce bottom paint).
There are reducers for bottom paint, but these are used for spray applications, extremely hot climates (90F+) or if you applying to wood. (bottom paint should always be reduced by 20% for the first coat of bottom paint on wood.)
When used for these applications, please follow the manufacturers’ guidelines. (typically thin bottom paint no more than 10% unless barewood (20%) per application)).
Don’t waste your money on bottom paint reducer unless your application meets these conditions. Otherwise, you are just spending more money thinning your bottom paint and waiting for it to dry! Remember that bottom is not like house paint, thinning bottom paint means that you are actually spreading less bottom paint over the surface. Bottom paint needs to be thick enough to repel growth as the paint film wears away over time.