Terrapins are strong and destructive animals, so under no circumstances use a glass encased model - if it is accidentally shattered electrocution could result. Use only metal or toughened plastic models and situate these behind a protected area constructed by fixing extra pieces of glass plate to the tank using aquarium seal or bonding agent - obviously, water must flow unimpeded around the heater. It is a good idea with any animal electrical installation to employ a safety earth circuit breaker system - these can be obtained from most electrical shops at moderate cost and can do much to improve overall safety standards. It is also possible to heat terrapin tanks using underfloor heat pads - special types suitable for this purpose are available from aquatic suppliers. These do have the advantage that they are totally secure from interference by the terrapins and in addition an external electronic thermostat can be used giving improved accuracy and yet higher operational reliability.
Dirty water is a sure recipe for inducing bacterial and parasitic diseases.
A dirty terrapin tank is also extremely smelly and not a pleasant addition to the household. Regular water changes are one way of achieving this, but the process rapidly becomes tedious in the extreme. The practical solution is to employ a motorized filter system which will reduce the frequency with which total water changes are necessary. Undergravel filters Undergravel filters can work very well, but do require a large surface area, low stocking density, and well oxygenated water. Larger tanks should be fitted with a powerhead in place of the airlift.