What’s causing this water clouding?

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Can a pH drop in the water column cause mineral salts in the water to fall out, and result in cloudiness? Neale Monks answers…

I’m using remineralisation salts on my RO water. When I do a water change everything looks fine but within a few hours the aquarium runs cloudy. I have bogwood in my tank and I’m wondering if the pH drop in the water column is causing the mineral salts in my water to fall out, resulting in this cloudiness. Is this possible?

MARTIN STEELE, VIA EMAIL

Neale says: It is true that some salts are more soluble at one pH level than another. The classic example is the formation of stalactites and stalagmites in caves. Water that is saturated with carbon dioxide rises towards the surface, and as it warms up, the carbon dioxide diffuses out, causing the pH of the water to rise. This, in turn, reduces the solubility of calcium carbonate, which precipitates out as limestone. Whether an acidic pH or a basic pH dissolves a given mineral ion better will vary, but realistically, the pH changes in fish tanks tend to be very slight. So, while theoretically what you think you’re seeing might happen, it would require a pretty steep pH change to actually occur — say, from pH 6 to pH 8.

There are several sorts of remineralisation salts on the market. The most common ones are for Rift Valley cichlid tanks and for Discus aquaria. The former are generally some sort of mix of bicarbonate, carbonate and sulphate salts that exert the necessary osmotic effect on the fish while also fixing the pH at around 8.0. Discus salts are more varied, but often include phosphate salts that fix the pH around 6.0, while providing traces of other minerals that help to keep fish healthy. On its own, RO water isn’t suitable for keeping most tropical fish but these remineralisation salts turn it into something that’s suitable for keeping hard or soft water species as the case may be.

The problem with many of these salts is that they’re not terribly soluble. The same issue exists when using artificial seawater mixes, and many aquarists recommend stirring them into buckets several hours, or the day before, a water change. Given time, and suitable agitation with an airstone, the salts will eventually dissolve completely, but stirring them directly into a bucket of water moments before you add that water to the fish tank doesn’t always work. Undissolved minerals will float about for a while, and eventually settle out on plants, rocks, or whatever, which can look unsightly.

It’s much easier to keep fish suited to your local water chemistry, in which case regular water changes will help fix almost any possible problem at minimum cost. If you must make water chemistry changes, make the smallest possible changes you can get away with. In places where the water is ‘liquid rock’ simply mixing hard tapwater 50:50 with RO or rainwater will result in around 10˚dH, pH 7.5, which is absolutely fine for the vast majority of community fish, unless you’re keeping or breeding some sensitive species,  Such water won’t need buffering or remineralising, and even Amazonian fish are more concerned with the hardness than the pH, and should thrive under such conditions. You shouldn’t notice much of a change in pH when using bogwood, even in generous quantities, as the carbonate hardness will likely be around 5–10˚KH.

If you must use only RO or rainwater, with remineralising salts added, thoroughly dissolve the salts the night before, and leave an airstone or small canister filter in the bucket to keep the water turning over. Test the carbonate hardness: this is what stabilises the pH in hard water systems, and below, say, 3˚KH, the pH can be very vulnerable. Discus salts should buffer against pH changes, but these generally assume that the tank will be lightly stocked and rigorously well maintained.