What if you were to get some procambarus clarkii, or the common crayfish that is sold on aquatic arts dot com. The freshwater crayfish comes in a motley of colors and will make for an interesting pet that will fill you with entertainment for hours on end! Procambarus alleni or the electric blue crayfish is also fun and an exiting color. I always thought clarkii was the more hardy of the two varieties. I have two ghost crayfish which is a rare color morph of the procambarus clarkii species that is blue-ish purple with red and white color formations. These are interesting little critters for your home aquarium and in a 42-gallon aquarium you could just have a few, maybe three with plenty of hiding spaces. Good luck choosing the right fish for the job!