My regani male made it to about 5". I would highly recommend not keeping them in 20 gallon tanks. They may be dwarf pikes, but they are still aggressive toward each other and my male harassed both of my females constantly. They started out in a 55 gallon with a bunch of caves and fake plants, and even that wasn't enough to stop the constant chasing and fighting between the male and the two females. I eventually moved them into my 150g community and they actually seemed better there.
There's this misconception that small fish are best in small tanks, but when you take into account the personality and aggression of a fish, this just isn't always the case.
I have a male regani that has wiped out more fellow reganis than I would care to mention, even in a 75 gallon. I made my girlfriend put our latest reganis in separate tanks for now and I will never put another dwarf pike near the one In our 75.
From DJ/MC Decker