What you are trying to do is not difficult, but it will be more work than a stat per tank and may not cost much less. However, it will give you a readout of temperature for each tank. Here are some suggestions:
1. Temperature sensors. There are lot of different types available. The DS18B20 is popular and has the advantage that you can connect several of them to a single Arduino pin. However, getting them to work when you have long wires between the Arduino and multiple sensors can be tricky. See http://www.maximintegrated.com/app-notes/index.mvp/id/148
for some advice. Alternatively, you could use analog sensors such as LM35 or TMP35, which can work over long lead lengths, but you would then need to add analog multiplexers such as CD4051 if you want to read more than 6 of them.
2. Controlling the heaters. For controlling 25 watt lamps (or preferably, power resistors instead), power opto triacs would be suitable. The maximum rating you can get easily is 1A, for example http://www.farnell.com/datasheets/1244300.pdf
. If the heater current is more than this then you will need SSRs. You will need one per tank.
3. Display: either a character LCD or a graphic LCD is easy to interface to an Arduino.
4. Controls: I suggest two rotary encoders, one to select the tank and the other to select the desired temperature.