Why is that every thread that Whitey posts in turns into the useless banter, in a hi-jacked thread, where he constantly attacks people? Sorry, Debbie...he's like our resident pain in the keister. We've just all learned to put up with him.
Debbie,
It's really difficult for a 25gal. to be turned into an aggressive tank. Aggressive fish are messier eaters, therefore requiring heavier filtration. And, there are not too many choices, since most will outgrow the 25gal. tank limit.
Dwarf Lions, no matter the species, would best be kept in a 40gal. or more...most will reach 6", in length, at least.
Anglers/Frogfish are a good candidate, for a 25gal. but require heavy duty filtration and are extremely sensitive to nitrates and copper.
Smaller species of Scorpionfish would be ok, but the problem is that 99% of fish stores either dont carry them or mislabel them, when they have them in stock. You could end up with a Scorp that'll max out at 3" or one that'll max out at 15"...it's a tough decision.
There are also Mantis Shrimp. Most people hate them and want them dead, especially in their reef tanks, but I'm a huge fan and have kept many over the years.