several types look very similar, especially at the smaller sizes. and they do change a bit in different life stages. i have never seen a diamond at the 20-26" range yours looks like its in. ive only seen juvis and adults
im no expert so dont quote me, but my vote says its a white sturgeon (Acipenser transmontanus) sometimes called "california sturgeon" or "lake / river sturgeon" here in LFS, and they are fairly common for a 2 month window each year
they are typically captive bred by the DNR / fish and game agencies, and some make their way into the aquarium trade. they are typically put into aquarium temp tanks at a very young age, and then tend to do better than other species in the warmer water long term (very limited success of people actually keeping them alive more than a year or two, so its hard to say)
to the best of my knowledge, all diamonds are bred overseas, and for the last 5-10 years they have almost impossible to find in the US.
a guy in florida has been selling them for the last 12-16 months, and my understanding is that the only way he was able to get them into the US was with a research permit of some sort.
when he brought them back in several vendors jumped on the bandwagon and started calling theirs diamonds too because they could charge more money and most people dont know the difference, and will kill them before they ever figure it out
here's a thread, these are 100% diamonds -- if yours didnt look like this as a juvi, it isnt a diamond.
http://www.monsterfishkeepers.com/f...hipping-or-pick-up&highlight=diamond+sturgeon
that all being said, no matter which species it is, i would highly reccomend adding a bubbler or some serious surface aggitation to that tank so its getting plenty of O2 in the water, especially at the higher temps