Right, sorry for all the questions, so wanted to get it right before I suggested anything.
Since youāre closing down the 125 and aqua one filter.
Move all the filter media from the aqua one to the 307, cut the sponge media if you have to, to make it fit into the trays and filter, add all the ceramic noodles, bio balls or whatever other media there is, all of it into the 307 in whichever order that Fluval recommends on the instructions.
And if there is still space in the new filter, add sponge media, add new ceramics with the established media to fill it up. The more media, the better.
Just donāt squash it all on too tightly if you get what I mean, just fill it without going OTT.
You may well get a mini cycle but the established bacteria in the old media will quickly form larger colonies to establish all over your new filter media, filter housing and tank surfaces.
And the mini cycle should not take long to do, anything from a day to a week usually.
Just keep an eye on water parameters and do large water changes anytime you see ammonia over 0.25ppm and/or nitrite over 0.5ppm.
Your plants should help a bit as well so all good imho to do all that.
Addition of floating plants will help a bit more as thatās a fast growing plant and sucks up loads of nutrients from the water column helping your ammonia and nitrate levels, so thatās good advice from
@HoldenOn.
By the way, I am a fan of Fluval tanks and filters, I have had a Roma tank before, quite nice and does exactly what it says on the tin and not had a 307 external yet, Iām interested in buying one of those maybe for my new 180 Juwel Vision tank or maybe get a Eheim 350 external, not decided yet.
So if you like the 307 filter and could let us know the pros and cons of that, that would be terrific!