I am considering getting my dad something for his tank for Christmas. It is an 8 gallon cube saltwater tank with an oversized Aquaclear HOB filter. It just finished cycling and currently only has a super tiny locally caught cocoa damselfish, a small hitchhiker pistol shrimp from my tank, and some small blue legged hermits and astrea snails. Does anyone have some suggestions for some cool creatures I could get him for Christmas, preferably around $100 or less? I know at that tank size there's probably not room for a second fish, but I'd still appreciate any invert recommendations. His nitrates are considerably high (>50ppm) so it would have to be on the hardier end of the spectrum.