This is long, as I want to try to give any info that might help.
I have a Fluval Spec V with my Betta and an onion snail. It has been going for several months. The filter compartment has space for a bag of biosphere balls, carbon and the sponge. I took out the carbon when I started it and added a second biosphere bag. I also have one on top of the sponge that I added a couple months ago. So, it has 3 little bags of spheres in the filter compartment. The numbers in this tank stay really steady at Ammonia 0, Nitrite 0, Nitrate 5-10. It is planted.
Then I started a 15 gallon in October for a community tank. I used the oldest bag of biospheres from the 5 gal. in the new filter, and replaced the bag in the 5 with a new one. I also used filter squeezings, and did a gravel vac and used the mulm to start up the 15. It cycled really fast. Numbers in it have been steady also, same numbers as the 5 gal. I would say it is medium to heavy planted. I just added a 2nd filter to it last week. I keep the filter on low, and it was not creating enough gentle flow, so I added a second one.
I stocked the 15 with 6 male cherry barbs to start. Then added 6 pygmy cory and an onion snail. Left that a few weeks, and 2 weeks ago, added 6 neons and some cherry shrimp. I'd been noticing some chasing among the cherry barbs - but didn't really think anything of it. Then I had 2 cory come up with wounds on their side. I moved them to a kritter keeper and medicated, but I think I caught them to late, and they didn't make it.
A week or so ago, it looked like the biggest barb was defending territory. A few days ago, when I did the water change and gravel vac, I rearranged things a bit to try to disrupt the behaviour. Yesterday, I noticed that same barb, and one of the other barbs have staked out about 1/4 of the tank each, and wouldn't let anyone in it. Lots of frantic chasing. I watched and watched to be sure. So, I thought I'd remove the cherry barbs, put them in the 3 gal (approx) kritter keeper that I keep as a hospital tank, and see if things settled down.
They did. Even the cherry shrimp were out last night. There is still one barb in the tank that I could not catch. He's the smallest, and most mellow of them, so might leave him in there for now. It is not as colorful as the others, so I've even thought it might be a female. Was always being chased by the others.
I have a used 10 gallon tank that a friend gave me. It has filter, light, heater etc. Just needs a good cleaning. I have enough gravel left from doing my 15 gallon, and they had some fake plants - so I plan on setting it up later today. Just got back from buying a stand to put it on. I had planned on setting it up as a QT tank, just keep plants in it when no fish. Figure I should put the barbs in it.
Since I have a biosphere bag in the 5 gal. I can use in the filter, - do you think I can go ahead and set it up, let it settle a day, then put the barbs in it? I just cleaned filters and did a gravel vac a few days ago, so I don't think I should do that again so soon. I don't plan on making this a fully planted tank yet - but will plant it as I have trimmings from my other tanks. I do have some crypts that I could pull out to put in though to start off with.
I think my options are
•set up the 10 gallon, let it settle a day, then add the barbs, hoping there is enough bacteria in the spheres, and on plant, that it won't go through a traditional cycle and all will be well...
•leave the 5 barbs in the little Kritter Keeper - which does have a betta bowl heater, but no filter - and do large water change every day while the 10 gallon cycles using a standard fishless cycle (using ammonia)....
•put the barbs back in the 15 and hope no one gets injured or stressed out till the 10 cycles.
I'm thinking if I use the spheres and mulm and a few plants from the other tanks, and carefully watch ammonia, nitrite, nitrate - doing water changes as needed - that I can go ahead and set it up and put the barbs in it.
I'm sorry I listened to the fish store person who told me the barbs would be "just fine" in the smaller tank. I think they need a much bigger tank to be happy, but I do not have a place for anything larger than the 15. This was a very well known specialty fish store, and an experienced employee. She originally sold them to me as nano fish for the 5 gal. Grrrrr. I learned a lesson - no impulse buys without lots of research.
I had a tank as a child 40 years ago - but still new at the "modern" fishkeeping.