Originally Posted by dylan94
I was wondering, if I want a low pH and very soft water and I am planning on using a reverse osmosis unit, can I just make the tank water pure RO water or do I have to absolutely add regular tap water. I have heard of cases where 0 hardness can lead to random pH drops and the water needs somewhat of a buffering capacity. Is this true or can I fill my tank with 100% pure water. Also, if I do need to add regular tap water do I have to get the exact measurments of water ratio or can I just do water changes by just adding RO water and then a bit of regular water. EX: I do a 50% water change. I fill up to like 2 inches below the top and then fill the rest with regular tap water?
I would suggest that this depends upon the fish you have in the tank, both what they are (the species) and if they are wild-caught or tank-raised commercially.
With no carbonate hardness to buffer the pH, the pH in an established aquarium will lower fairly quickly after the initial "settling in" period. My tap water is <1 dKH and <1 dGH, and the pH is 7.0 [they put soda ash in the water to keep the pH at 7, it is around 5-6 naturally]. My 70g and 90g aquaria have no buffering agents added, just straight tap water. The hardness is zero (GH and KH) and the pH is 5 or perhaps lower, my test kit only goes to 5. When I do the 50% pwc every week, the pH rises slightly, but not above 6; the lowest test kit I can find indicates 5 as the lowest colour, then 6 with nothing between, and the water is never at 6 so I can only assume it remains close to 5. These tanks have mostly wild-caught fish from Amazonia in the 90g and SE Asia in the 70g; they are vibrant, active, always spawning, so I must conclude they are happy and healthy.
In my 115g I have half a cup of dolomite in the filter; the hardness is steady (for months, even years) at 1-2 dGH, and pH is 6.0 to 6.4 with the usual diurnal variation that occurs in planted aquaria. I have a mix of wild-caught and tank-raised fish from Amazonia in this tank. Spawning is regular among most all the species.
All of the fish I have come from very soft acidic waters. We know that in their native habitat the hardness is so low is cannot be measured, or at the very most may be somewhere between 0 and 1 d, identical to my tap water. The pH of many of these waters varies from 3.5 to 5.5 depending upon the locality. I am speaking of the specific habitats of the species I maintain; there are streams in SA with harder and slightly basic water, but I always thoroughly research the habitats of any fish I intend to acquire so I know I can provide exactly what they require, and those in my aquaria are suited to my water parameters.
Fish and plants require minerals. Fish food provides some of these to fish, and the resulting waste provides some to the plants. We also use liquid fertilizer, root fertilizer, and sometimes enriched substrates, all of which add minerals for the plants. If you have hard or relatively hard tap water, mixing some in with the RO water for the initial filling of the tank can do no harm, and probably would benefit. Monitoring the pH weekly would indicate any lowering as would be bound to occur over a few weeks with a very low hardness, and then replacement water (during the weekly pwc) could be mixed to maintain a stable level, such as I have in my 115g. This avoids any sudden shifts, which is the danger; once a tank is biologically established and mature, it has a tendancy to remain stable even with the influx of water with a not too significant higher degree of hardness and pH. I would base this on the fish and their source.