unpressurized tank temperature monitor

Any suggestions on a reliable /inexpensive temp monitor? I saw there are compost temp probes in various lengths that could be inserted through the insulation on my top and stuck into the water. could slide up and down to adjust but at 2' would be mid way into my tank and should give a decent average temp? would not require electronics / batteries etc. Anybody ever use one of those? Any suggestions are appreciated.

When I had a tank like yours I used a dial thermometer with remote sensing bulb like this: http://cgi.ebay.com/Temperature-Ind...faultDomain_0&hash=item1c47d34b2#ht_551wt_905

The one in the ad is not the right temp range, just showing you the type I am talking about... I bought it at a local Webb Store...