First of all, I go by the rule "Happy wife, happy life", so absolutely have power for everything that your wife needs, wants, or uses, at all times.
I was surprised at the original generator selection when I first read it, but it was several weeks since it had been purchased when I saw it, so I held my tongue. I agree with iluzhun about sizing the generator to supply the whole house, and not just the fish room. Two generators is twice the maintenance, and almost twice the square footage of a unit capable of supplying the whole house.
I just went through upgrading the generator and central UPS for my server room at work, going from a 35kW to 100kW generator. The footprint of the larger unit is only about 20% more than the old unit. We went with diesel on both units for two reasons, the first being completely independent of public utilities in case of emergency, and second being the significantly larger natural gas supply that would be necessary in a leased space.
The challenge with my project was finding electricians that were familiar with generator installation, sizing, etc.. I contacted 4 that were recommended by the construction manager for the property, along with 3 others. Out of these seven large commercial electricians, only two were really able to handle the project. This might be what is coming into play with your project--the electricians may be great in other things, but need some help with generators.
Your desire to isolate the fish room and main house power can still be achieved while protecting both with a single generator. Two transfer switches can control a single generator, as well as having a separate power feed from the generator for each panel. This requires moving into a commercial generator, but you are already into the price range of a commercial system by having two generators. Commercial systems also have far better engine controls for tight voltage output, more options on exercising the engine (frequency, with/without load, duration). You could also link it into your tank controller to shut off non-critical loads when on generator power. If you are running on generator power, you don't need tank lights. You need water and air circulation. Turning off your lights reduces the need for cooling too.
Which brings up some thing for your SOP's, make sure that you run your generator on load regularly so you know that the tank will continue to run properly on generator power. You don't want to find out when your utility power is out that your generator doesn't work.