Thats a nice looking frame you have there. Did you and your husband design it yourselves or did you use the design from somewhere else? The design looks like a combination of different ideas I have seen on the Internet.
The joint is a 1 ¾ wood dowel plus 2 3inch long drywall screws topped with woodglue.
Although this joint won’t hold any weight, I wanted it to be massive.
While that joint won't be under a tremendous amount of load from the tank, your method of joining the wood makes a lot of sense. The joint comprised of butting 2 x 4s could have a tendancy of pulling apart due to the tendancy of a 2 x 4 wanting to twist. Your joint should remain sound and stay together without any problem.
What kind of 2 x 4s did you use? They look a lot like Douglas Fir.
Pre cutting all of your pieces is also a great idea. It allows you to check to make sure that your measurements between the pieces are precise which will help to make a square and level stand.
I would like to see some full stand shots as well.