Ph Rise 15 Days Into Cycle

Did you dose with bicarbonate of soda? It does raise pH as well as KH. Don't worry at this stage, the filter bacteria grow faster at high pH. Once the cycle has finished you will need to do a huge water change to get rid of the sodium part of the bicarb which will reset your pH as well.
You might want to think about when you have fish. Your KH is very low. You will need to do pretty big weekly water changes without fail (I came here asking for help some years ago when I got a bit lazy over water changes and my pH dropped though it has been fine with just 30% weekly water changes) or add some crushed coral/limestone rocks which will dissolve slowly and raise your KH or even add a small amount of remineralisation salts, the kind used with RO water to boost your KH a bit.

It's not the KH of the water that constrains the type of fish you can have but the GH.

GH is general hardness but despite that name it only actually measures divalent metal ions. For our purposes this is calcium, with maybe a bit of magnesium. Some bodies of water have only small amounts of calcium and fish in it have evolved to cope with that level; other bodies of water have a lot of calcium, and fish there have evolved to cope with a lot of calcium. Some fish can live in a wide range of GH, some need specific levels; rams need soft water (low GH) and Rift Lake cichlids need very hard water (high GH).
It is the GH figure, usually called just hardness, that you need to look at when reading fish profiles.

pH is connected with hardness but not directly. The type of minerals in the water cause the pH value. It is theoretically possible to have hard water and low pH, though that rarely occurs in nature. I have softish water and pH 7.5. pH does affect fish too but not as much as hardness. So I can keep fish that prefer soft acid water more easily than those that prefer hard alkaline water.

KH comes into play in stabilising the pH. It doesn't affect fish directly, just by stopping pH swings which are not good for fish. And of course low KH affects cycling as the filter bacteria need carbonate in the water to multiply quickly.


I've just gone back through your posts and see that your water company says your GH is 2.8 german deg/50ppm. That is classed as very soft. Unless you add something, you will be restricted to fish from soft water origins such as the Amazon. Crushed coral will raise things a bit, but not much as it is not very soluble. I did an experiment a while ago, where I put some crushed coral into a tub of water and left it for a week. I also left a tub of plain water alongside as a control. After a week, my pH had risen by 0.2; GH by 2 german deg and KH by 2 german deg. As you can see, not a very big effect. You might be better off with remineralisation salts if you want to keep fish from anything other than soft water regions. The amount you'd need to add would depend on just how hard your water needs to be for the fish you want to keep. if you stick to the softish end of the spectrum, you wouldn't need very much. But if you want to keep Rift Lake cichlids, you would need to add a lot.
Look for Kent RO right, and Tropic Marin remineralisations salts - the tropical one not the marine one.

Since Tommo has an external filter, that is indeed the way to use the crushed coral. Since I have only internals I always forget about that.

Thank you both for your replies. I think I will be using remineralisation salts in the future, especially after seeing the figures of water testing after crushed coral was added. Doesn't seem to rise level too much.