...Im wanting over 10 year life spans on all the fish and am trying to do this by doing a slower growth rate to full growth. Weird theory?...
Not at all. There is a large and varied scientific literature that demonstrates that growth rate and lifespan are inversely correlated. This includes all kinds of creatures, from experimentally tractable 'model organisms' like
Caenorhabditis and
Drosophila, to fishes and mammals. And we're not talking about pathological states that can be induced by under- or overfeeding (starvation and obesity, respectively), but about animals whose diets reside within their 'normal' ranges of caloric intake. Of course, there are many other ways to influence growth rate,
e. g. ambient temperature for ectothermic animals, and these too have the predicted effects on lifespan.