It’s hard to explain without a diagram, but let me try:

Buffer gate (top right): 5s
Filter gate (top left): 5s
Buffer gate (bottom right): 10s
Buffer gate (bottom left): 5s

It’s been a while since I designed this so only vaguely remember, but you may be right in that the water dissipated after a while. I know I hit that problem a few times and had to tweak. However, as I look at the saved game, I don’t see any source of water in that room, so hmm, maybe I did get this right so that the water didn’t disappear ….