wrap a more than normal amount of ptfe tape on the barbs and put silicone sealant at the top of the thread, screw it in then mroesillicone around the outside where they meet. araldite might be better, but it owuld be permanant, which you wont want.
All of the plastic barbs Ive used apart form teh ones with my ehiem had annoyingly thick walls, especailly on the 90o ones. I recommend you to use a normal barb, of the right size and brass, and then use a copper pipe and copper 90o. It would mean a bit of tubing between them but it would also be about 1/3 or 1/4 as restrictive as plastic 90o's. Pump inlets need plenty of water or you get lower flow rates...
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