With regular K2SE settings a single Spidertron should at most cost you 30 fish. 10 for the power source that goes into making a Spidertron and another 20 for the maximum 2 power sources you can fit into a grid. You can relatively easily fish with construction robots and a decon planner. So unless you have disabled natural bodies of water there should be more then enough fish around to build an initial fleet of Spidertrons.
Also if you are really really desperate for more fish. Launching a stack of space science packs does generate an equivalent amount of fish.
I may consider adding an option to change out the fish in the recipe in the next version but no promises. I will definitely not add alternate recipes as that is just unnecessary bloat.
