I have made a number of water proof temperature sensors using DS18S20 1-wire sensors. They use a TO-92 case (looks like a transistor).
I have used two approaches depending on the pressure, either I form PVC plastic around the sensor (like clear 1/4 water line for the icemaker) which has the advantage of fitting in plumbing fittings so that it can be inserted into a pressurized system, or I use heat shrink tubing for atmospheric pressure - which would be what you need for an aquarium.
Here is what I did for my pool. (I had several leaks before finding this method; U tube both ends above maximum water level, double layer to avoid leakage.)
1) Get a suitable length of cat 5 cable.
2) Get four diameters of heat shrink tubing
    a) 36 or 48 inches of 3/16 (for a depth of 17 inches, or 23 inches into the water
    b) 36 or 48 inches of 1/4
    c) 3 inches of 1/16 (cut into 3 one inch sections)
    b) 2 inches of 1/2 tube.
3) strip back the cat 5 cable removing all but the three wires you are going to us for the 15 inches (21 inches)
4) Place the 3 heat shrink sections over the ends of the wires and move them up away from the solder joints
5) Solder the wires to the DS18S20 - make sure you use the "pair" for data and return and the odd wire for power.
5a) optional - add additional DS18S20s along the wires at intervals with insulators
    (For example I have 4 sensors in about 8 inches, two within an inch of the target water level for the pool, one 8 inches into the pool, and one above the maximum level of the pool water, with this I can measure pool depth, pool temperature.
6) Slide the 3/16 tube inside the 1/4 tube (e.g. double glove)
7) Slide the wire with the sensors into the tubes - upto half way and far enough that the tube overlaps the CAT5 jacket by about 2 inches
8) Heat shrink the tubes onto the sensor
9) Now fold the tube in two so that the sensor is at the bottom and the tops of the U are both together (although one has a Cat5 cable coming out of it)
10) Slide the 1/2 section over the bottom of the U and up to the tops of the U, fold the empty top back onto itself and cover with the 1/2 tube and apply heat to shrink.
11) Put appropriate connectors on the other end of the able and try it out.
 -----Original Message-----
From: misterhouse-users-bounces@lists.sourceforge.net [mailto:misterhouse-users-bounces@lists.sourceforge.net] On Behalf Of TimC
Sent: September 26, 2006 7:49 PM
To: misterhouse-users@lists.sourceforge.net
Subject: [mh] Measure water temp

I'd like to build an interface to read the water temperature in an aquarium.  I do have experience with the 1-wire sensors, but only the button style temp devices which are obviously not water tight.

Any pointers would be appreciated.