Boraras turning black

[GaspingGurami]

I noted that the OP only topped off the tank to replace evaporated water.
This causes the disolved solids to increase and fish waste too.
AS they say, the solution to pollution is dilution. Regular large water changes are necessary.

The other way is through removal of solid waste before it can dissolve, through frequent filter cleaning, and plant pruning. This tank is filtered by an Eheim 2222 and a Hydor Prime 10. I needed 2 filters because water circulation was a problem with the dense growth of moss and other plants.

Boraras spp do best at at pH below 5.0 and extremely low dissolved solids. Use rain water or RO water with just enough minerals to keep the KH at about 2 and a GH of 3.

Today, 4 years down the road, the same Boraras are still around. Plant mass had been quartered as I get complaints from the family that there is nothing to see with all that plant growth obscuring everything. Now with more light, their colours have returned, so I still think that their colour blackening was due to adapting to dim dark conditions of that time, as I don't have black ones in the brighter tank now.

These are somewhat extreme conditions but many fish need them. Most of the wild Betta of the coccina complex and Chocolate Gourami are examples.
Checkerboard Cichlids, Dicrossus filamentosus is another example.

I've some questions to ask because I've not succeeded in keeping Chocolate Gouramies and Sundadanio axelrodi for longer than 9 months.
1) shouldn't pH go down if fish waste is allowed to accumulate? I always thought mulm sours the water.
2) Although these factors go hand in hand, which of these 2 groups are more important to these softwater fishes - low pH or low KH and GH?
3) my KH is 2DKH (new and old API testkit). pH hovering from pH6.0 at first light to pH6.8 at last light can be attributed to plants using up the CO2 in the water in the photoperiod. I've not added any fertiliser for a long time now, neither is CO2 injected. There are no rocks beside lapis sand. In short, there's nothing to harden or soften the water besides the fish food (which I've not tested). Can it then be possible for pH to go down to pH6.0 at first light (that works out to be 59ppm CO2!!!) without supplementing with CO2? Or could this be due to the acidic mulm buffering the pH that skewed the reading? (Even tested pH with all electricals off - just in case theres a power leakage to fool the pH pen).
4) My last request for guidance, that I've not been able to achieve is how to get the pH down to pH4.0 without resorting to using pH down, CO2, acid buffers or other chemical soup? At KH 0.1, it takes 300ppm of CO2 to bring the pH down to 4.0. What can I do to achieve pH4.0?

Thanks for replying.

[TyroneGenade]

What is the NO3 level in the tank when the fish turn black? At high NO3 level bacteria in the gut of the fish can convert the NO3- to NO2 which will bind to the hemoglobin and turn it blue. These blue and very dead blood cells can get stuck in the capillaries of the fish and cause black flecks to appear.

That the black disappears when the plants are trimmed suggests NO3- to me because with pruning comes renewed and vigorous growth which will suck NO3- out of the water.

The fish don't die from the NO3-/NO2- poisoning because they are so tiny which allows the passive diffusion of O3 through the system to sustain it. However, the hemoglobin now also can't carry CO2 which means the blood of the fish turns acid causing the hemoglobin and other blood proteins to precipitate in tissues to far removed from the gills.

The rise in tank pH is due to the plants taking up H+ ions to synthesize sugars. Your fish might have been dead were it not for the high pH which could prevent all their blood going acid.

Don't mess with the pH. The fish are happiest at a stable pH rather than one which needs constant tweeking.