Great points and I completely understand. One question though, and correct me if I'm wrong, but one problem with under gravel filters is that they use the gravel as biomaterial and if not properly taken care of, the BB take up too much oxygen and so you can basically choke out the fish. Therefore wouldn't it be difficult to do heavy stocking as the fish would not have enough oxygen or would I just have to use bubblers to try and to increase the oxygen on the water?
Hello; First the under gravel filters (UGF). I must have missed any parts of the thread about using UGF. I have, however, used the UGF's many times over the decades. At one time they were the only filtration I used in tanks. I have used them in heavily stocked tanks and in tanks before I discovered the value of regular water changes. It was also a time of my overfeeding. While there were some issues in those setups, oxygen depletion was not a problem as I recall. I ran the UGF's with bubblers and often extra air stone bubblers. I also used a slightly coarse gravel substrate, bb sized and larger, which allowed some flow thru the gravel bed. I also kept live plants the tanks.
Next; There are at least two types of filtration. Mechanical is the type where the water is pumped thru some sort of media that traps the bits of detritus and can remove it from the water column. These are the HOB's, canisters and such. These filters do double duty as they also become a physical surface for the bb to colonize and the bb do the bio-filtration. The bio filtration is the only thing the UGF's actually do. The bio-filtration in a cycled tank changes two very toxic products thru the actions of at least two kinds of bacteria so we wind up with a less toxic product (nitrate). Live plants can use up some of this nitrate and we dilute the rest with regular water changes. So yes the cycling process needs oxygen. I also suppose that tanks can have too much fish biomass and that oxygen could become a limiting factor in a heavy stocked tank. Such heavy stocking will have other negative issues as well.
last; The use of plenty of bubblers can help make sure there is enough oxygen in the water of a tank. I think of such heavy stocked tanks as tightrope tanks, always on the edge of some sort of serious issue.
Back to what I had interperted as you plan. I did not envision using fish to provide the ammonia for feeding the bb in the shop tank. I was thinking of using the liquid ammonia from a bottle as the ammonia source. If live fish are used, then when you robbed the bb loaded media to do a quick setup in a client tank it would likely result in an ammonia spike in the shop tank. Bad for the fish. Using bottled ammonia would, it seems to me, eleminate that issue.
Another benefit of using the bottled ammonia might be that the shop tank could be a plant grow out tank. At least that is where just thinking about it leads me.
here is a link i use for reference.
http://www.americanaquariumproducts.com/nitrogen_cycle.html#nitrification
