Hi RG,

We used to live in Piedmont and my Dad built a pond with his Dad in
1935 in Berkeley. In all our years there, we only saw frost once.

With 18 gallons, you have an outdoor aquarium. Others will no doubt
have a better set of recommendations than I s to the best filter for
today. When I had an outdoor 20 gal tank (in Florida), we used a sub-
sand filter hooked up to an oversized (for a 50 gal tank) external
filter in which we had a filter sponge. The sub-sand was set up to
help clean the tank and the sponge was easy to pull, squeeze out and
replace.

The crucial piece for us was to protect the electrics. If I had it to
do today, I might use a powerhead in the tank so I would only have to
protect the plug.

Let us know what you decide.

Jim