The flex tanks have a lower "overflow." I've always wondered why, but maybe they're anticipating something like this happening. I'm with you on the heater too, looks identical to a jagerIt doesn't even seem like the water is high enough to reach the overflow. I guess there may be a lower opening? Is the return pump plugged in and water circulating?
Can you pull the heater and take a quick pic too? I don't see how that isn't a Jager, it looks exactly like one from above.