You only need to put teflon tape on taper fittings.
These seal on the thread surface, so the tape helps form the seal.
If your fittings are parallel (or straight), they normally seal on the face by means of an O-ring and have no need of teflon tape.
Eheim pumps, for example, use a parallel thread and do not require teflon tape or sealant.
They seal by the O-ring supplied.
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