Best top off system?

[mark]

go the mechanical float valve (for furnace humidifier from HD, $12) connected directly to RO/DI. 6 years, zero problems, can't say I notice TDS creep, only second membrane since started (and believe probably changed out first when didn't have to), simple.

Yes there's a risk but the valve is continuously being flushed with fresh water and really this is the same valve used on furnaces. Figure there's a whole bunch more furnaces than fish tanks and can't ever say I've heard of someone flooding their basement because the humidifier screwed up.