Mate give us a few more details of your setup, such as what size / type of tubs, distance between the shelves, hanging or sitting tubs,is the system to use a stat, is it double or single tub per depth of shelf etc? If you give us this info we can give you some back that has a fair chance of being correct.
e.g. I use 65W for single depth and 35W for double depth, (due to the width of the heat tape), both obtainable from the herp shop here:
'MULTI-HEAT' FLEXIBLE HEATING ELEMENT
Available in a variety of sizes and watt densities to heat cage floors, shelves etc. on which cages/plastic containers etc. are to be placed.
Like the above heat mats, it is necessary to control the first two with a probe thermostat and/or rheostat and advisable, for accuracy, for the lower wattage element (ELE035S2-330) as well.
I use mine through a combination of stat and dimmer both also available from the herp shop.
Hope this helps.