In line follow the directions in the manual.... What are you having problems with specifically? If its because the hose is the wrong size for the heater use an adapter but be aware it causes a loss of flow.
The heater was previously on an aquis 1250, but have changed to aquis 2250, which has way bigger pipework, so just wanted to know if there is anyway i can stick with the external heater, as it says the 300watt heater fits any external filter on the market?