Hi there

As the title states why won't it prime? It's a Oase Biomaster Thermo 250, all connected up correctly and you push down the large 'button' on top of the pre filter to get it to prime, but no matter how many times I do this the water just won't circulate through.

Both tubes have got some water in but when I push the button the water moves a little then just goes back, it just doesn't want to go any further. The filter is on the floor so well below the water line, and I've tried moving the filter to displace any air but it just won't do it.

Is there anything else I can try?

Make sure you haven't got the intake and outlet reversed. Other than that, empty all water from the canister and pipes, then suck on the return pipe and that'll start it filling. Once it's full, switch on.

Inlet is connected to in, outlet is connected to out.

Each hose is 2 metres in length is that too long?

Sucking on the pipe will be a last resort!

2m should be OK. You don't have to suck any water, just a quick suck to start it syphoning into the canister

Gave the outlet pipe a good suck but didn’t realise there was still water in the tube so that was terrifying!

Luckily there was no dechlorinator in there because I’m fairly sure I swallowed a bit of it. Anyway it worked a charm let’s just hope I live to enjoy it!

You'll be fine Glad it worked.

It's been running fine - had a small leak issue at one of the connectors, which I've sorted but I've noticed it makes this strange scraping or rattling sound; it only lasts for a second or so but it does it quite regularly like every 20 mins or so.

I thoroughly checked the impeller when I set it up and it was fine and I've also gently tilted the canister to displace any air and no more air is coming out - is this something I should be concerned about?

Rattling usually means trapped air. If it doesn't settle, you'll need to empty the canister and prime again from scratch

God I don’t want to go through that again.

I’ve been listening out for the noise and it actually does it consistently, roughly every 20 minutes which is really strange. It can’t be the heater turning on or off because it is far too loud to be that so I’m not sure what it is.

Just wanted to update this with progress and a question...

It turns out the noise from the canister IS the thermostat on the heater, not trapped air, which is what I thought due to the consistency of the noise happening every 20 minutes exactly so I'm pleased.

My question though now is I'm fishless cycling with Ammonia and it's been 9 days but I'm yet to register Nitrite. I've dosed it to roughly 3ppm and I was expecting to at least see some progress by now. Is there a general timescale for Nitrites to appear?