Tank Maintenance
If your school is currently participating in our Eels in School programme you may find the following videos helpful. 

Clearing an airlock from the Filter.

If your equipment is making an unusual noise (this can sound like a loud rattle) then it is likely that some air has made it's way in to the Filter.  It's important to clear this as soon as possible in order that the Filter can resume functioning correctly.  If the Filter is left uncleared the creatures in the tank are not receiving clean water which can ultimately lead to loss but a long time airlock can also damage the equipment.

Changing the water in the tank

Ideally the water in the tank should be changed by about 1/3 of a tank per day.  Once the dirty water is removed clean water should be added but it should be at air temperature, having been left to stand in an open bucket for at least 12 hours.  This ensures that the majority of the chlorine has evaporated.