And, I am back in action with CO2. The local welding shop swapped the non-hydro'd tank, no questions asked. The tank they gave me has hydro dates going back to the early 1960's. I guess that says something about the durability of a steel tank.
Last night I also finished rigging the auto topoff/auto water change system. This is probably my favorite part of the system because I think I've managed to hit the sweet spot of "simple yet effective." I've done all kinds of ATO and auto water change setups on marine tanks, but the flexibility of not having to maintain salinity means I can combine the two on this system.
I don't remember if I explained it in detail yet so here goes. In the basement below the tank, there's the remnant of my old RO/DI unit. Basically, just the prefilters - sediment, and two carbon blocks. This ensures the water has no sediment and really bad things (chlorine) are gone. A standard poly line runs from this filter up to the bottom of the stand, through a chaise I use for plumbing and wiring.
Inside the stand, the water supply line goes through a solenoid. The solenoid is on a timer. After the solenoid, the line runs up over the rim of the tank and is secured to the return plumbing from the main pump.
Inside the filter box (old overflow box), one of the 1.5" bulkheads in the wall of the tank is plumbed to a 90 that points straight up, with a very short length of straight pipe. This upwards-facing pipe terminates right where I want the water line in the box. It's covered with a home-made grille to keep fish/plants/filter media/whatever from getting in. On the outside of the tank, this bulkhead is connected to a standard hose bib that has a hose on it, which runs outside the house to a drain in the yard.
The solenoid on the water supply line is on a timer that I can set to run for a few minutes each day. I haven't dialed this in yet, but the eventual goal is to set it so it runs JUST longer than required to account for topoff. When it runs, the water level in the filter box rises. Once it hits the level of the drain, old water drains outside the house.
With this design, I can account for evaporation, AND perform very small daily water changes, with nothing more than a timer and a solenoid. No float switches to run the ATO! No complications! If the solenoid sticks shut, I have probably 5 or 6 days before the main pump starts sucking air, no big deal. If the solenoid sticks open, no big deal, extra water just goes down the drain. If the drain partially clogs, well - it's got a whole day before the water supply runs again to drain the small "extra" water. If the drain clogs 100%, I will have probably a week or two before the "extra" water builds up enough to cause a spill. If the drain clogs 100% AND the solenoid sticks open, I've got probably 6 or 8 hours before a spill. These are acceptable levels of risk in my book, and way way less risk than most people with ATOs or auto water change functionality seem to have.
If I want a large water change, I can just override the timer and let it run for whatever interval I want. The water enters slowly enough that I'm not worried about matching temperature. I will NEVER have to deal with lugging buckets of water or hoses or any of the typical mess associated with water changes! If I did ever decide that I needed to siphon detritus from the substrate, I can just unscrew the drain hose and start a siphon in it, suck things out, then override the timer and let the tank refill.