I hear ya Steppy. Been there, done that.
Though, what I'm suggesting is not that you "give in" or anything - just the opposite. If he thinks he's so smart, then prove that you're smarter. To yourself!
If you guys argue lots, well first of all that's normal, but also, we humans have this stupid desire to say or do things that are meant to hit someone where it hurts - just to "win". If you make the tank the object, then you can't win! (Not under you circumstances right now.) Fight and argue over something else - but don't allow him to see how important this is to you. Yeah, take full control over the situation of the tank - but not out of spite, or he'll "try to show you how it's done", cause he's a guy and we guys have these things called egos. Let him "show you how it's done" over something else.
You deserve to have ego too. Just be smarter about how you use it. Picture a month from now (which doesn't help your poor little guys at this moment), where YOU are in full control of the tank, though Dad feels great becasue you guys "argued" about something else that HE thought was really important to you (which wasn't), and he "won".
(Some guy is going to hate me for giving you this info!)
Either way, you sound very independent and capable of making up your own mind as to what you want.
Stick to your guns. Just be "smarter" about it!
(Just my opinion. Could be way off here)