Complex sentence: Many aquarists view the use of feeder fish as cruel and unnecessary, arguing that the feeder fish is introduced into a small tank it has no chance to escape, and such a contrived situation cannot be considered "natural" in any meaningful sense.
Split into simpler sentences: Many aquarists view the use of feeder fish as cruel and unnecessary. Since the feeder fish is dumped into a small tank it has no chance to escape, and such a contrived situation cannot be considered "natural" in any meaningful sense.