Hi,
As title, got a new 70l tank that Im going to use to breed BN plecs. Im using one of the internal filters I have in my brackish tank, so will that give me an instant cycle?
Or cant I use it cuz it will have a bit of salt in?

It depends on the salinity in the brackish tank. If the SG is above about 1.005 then I would expect most of the bacteria to die from the change in salinity.

Can't you 'acclimatise' the bacteria back to fresh-water, Andy? or have the salt-water nitrifiers kicked in before that salinity is reached?

Just curious.

By acclimatising them it is hypothesised you are letting the different salinity bacteria die at a rate equal(ish) to the growth of bacteria at the new salinity, so any acclimatisation will likely take the same amount of time as a new cycle. Salinity needs to be moved slowly in brackish/marine tanks for the sake of the bacteria, not the fish.

Very interesting Andy. Thanks.

So you are saying that it is a different species of bacteria present as compared to fresh-water aquaria, even at a salinity of 1.005?

At what salinity do you expect the transition of bacteria types to start taking place?

From reading nmonks, it appears that once the specific gravity gets above 1.005 (quick note: salinity is measured in parts per thousand, the relative weight of water is noted as 1.005) you start to get different bacteria. I do recall reading that the AOB in marine conditions is different to FW. Nitrococcus springs to mind, but I may be well wide of the mark there.