does anyone keep an aquarium with a ph under 5

If so please contact me, I would like to pick your brain on your approach to water chemistry and stability of your system.

Wow I though my water at 5.5 was low. What are you planning on keeping?

That sounds hard.... er, soft rather, I suppose

I do not know how you could keep something like that stable, it seems like that would eventually even erode rock and then be buffered up to a higher PH

I'm considering working with a new project fish (dicrossus filamentossus) I read that they need very low ph to have viable eggs. that's exactly my concern "tank stability". I currently run my planted tanks in the low 6's and anything under that negates the bacteria nitrogen cycle, I don't really want to get in to an ro unite or chem dose my tank to make it as cost effective as possible.

so if anyone has experience dropping there tanks that low while maintaining stable water quality please feel free to chime in with your approach to lowering your ph.

Nitrogen bacteria have been found thriving all the way down to 3.8 pH, so that wouldn't be my worry. Usually to get the pH down that low requires practically no hardness, which means little to no buffer when the pH crashes due to organic material decomposing. I would think small, frequent water changes would be the key. Not sure if you can get it that low without RO water though. Maybe a canister filled with alder cones?

I wish you luck, raising D. filamentossus fry is one of the greatest challenges in the cichlid world. I've known nationally renowned apisto breeders that still can't do it.

The problem I see with using ANY natural material would be, as dwarfpike pointed out, the unpredictable rates of decomposition.

Since they seem like a small fish, you could try just very small tanks with RO and go buy RO water from somewhere and haul it in... that would quickly get to be a major pita though. But, if you had the setup small enough, it is do-able and would probably be the easiest way to ensure minimal PH swings and keep other parameters stable in such soft water.

The other thought I just had is this; maybe you don't need them to be at such a low PH all the time. They might not live in water that acidic all the time in nature either, and this may just be a seasonal signal that causes the fish to start spawning at a certain opportune time of year. If you can mimic that change at the right time, you may be able to induce spawning by temporarily lowering the PH enough to keep the eggs viable.

Branching from that thought, the next thing I'm thinking is that maybe the eggs don't need to be with the parents. Would the parents stay with them in nature, or would they move along/die off? If there isn't any parental care needed, you may be able to work out a separate egg incubating system that keeps the PH low so they remain viable, something much smaller than the main spawning tank.

Of course I know there are numerous complications with all these ideas that I'm not addressing, and also I am mostly talking out of my butt, but they are just ideas I thought I would throw out there so that they can possibly help you think of the best way to do your project.

you'll have to start with RO water to strip it of minerals etc.

you can add catappa leaves, alder cones etc to help lower the pH even more but you may need to use mercuric acid to get it even lower.(see research or techniques applied with Altum keepers)
generally speaking nitrifying bacteria begin to slow when the pH begins to dip below 7..at around 6 they are very slow and may become dormant.. below 6 they tend to shutdown completely.
keep in mind there is more studying being done about the type of bacteria that are actually involved with the nitrogen cycle. but at the low pH ammonia NH3 is generally converted to NH4 with the free flaoting H+ ions in the water.