Just gotta be patient on that one. I have a baby on the way too. So I am not adding any tanks now.
Just settling with the ones I have until probably next year sometime. By then the fish should be large enough in the 55 and 29s that they need to be put into my planned larger (375g) tank anyway.
Recently I have been working on upgrading rather than adding things. For example I just upgraded from the tetra heater that came with my 55 gallon (which still works perfect so will be a great backup) for one of the Fluval E-series heaters. This should help protect my fish from burns when they get bigger. I also just recently replaced my two HOB
filters on the 55 with a Fluval FX5. I'm basically trying to get most of the equipment together that I might need for the 375g before I get it so that when I get the tank I am basically ready to go, using all the substrates etc from the smaller tanks.
Maybe doing some smaller upgrades on your existing tanks (some new plants, perhaps?) would satisfy the urge?