Haven't lost a fish yet. My tank dips from around 80 to right about 70 during this process.
But what if you could hear them screaming?
Dunno, a small dip seems no problem, 10F diff seems a bit much to me.
sws - the water system pressure wouldn't matter if the end of the hose is open. The shutoff/valve would be before the heater if I understand correctly.
brogan - regurgitating a couple of ideas:
1) Slow down your water changes so the aquarium heater has a chance to keep the temperatures up. Small dips (2-3F) shouldn't be a problem. Add the water right where the heater is located so the thermostat switches the heater on quickly.
2) Skip the filter, use a regular faucet to add temperate water, add some Prime or similar to deactivate Chlorine/Chloramine.
3) I think the inline heaters would work if you refill the tank slow enough. Not a cheap solution. Or you could use an intermediate container (bucket) with heater(s) where you add the water and then transfer it from there, either via gravity (place it somewhere above the tank) or via a pump (bucket in the stand?). Fill the bucket, let it sit for 20 min to warm the water, then drain, etc.
4) Consider smaller, more frequent water changes. Like 20% every other day. This is where you start to think about automating your WC and making them daily 10%.