125 is way too small for an adult gurnard. They grow to like 18" long and way wider with their fins. Need more like a flat 300 gallon.
I second an octopus. An O. briareus or O. vulgaris would be ideal for that size of tank. Just fasten those lids down good! As long as you get a very small one it will be around for a year or more, grow pretty big, and unlike a fish, it can reach out and play with you.