The tap water pH is 7.3 according to my strips.

The tank is 100litre, with substrate (a white gravel type). Heater and filter but nothing else.

I found the ph does go up in the tank,for some reason
Mine is less from the tap too! But 8 in the tank

I just checked with my water supplier and the pH is really 7.3 so my strips seem accurate on that. The only explanation I can think of is that the substrate is alkaline and raises the pH.

Nice Malwawi tank. Are they breeding yet?

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If the Ph in the tank is higher than your tap water then there is something in your aquarium that is effecting it. It could well be your substrate.

I expect mine to be high,with using ocean rock
My malawis are breeding,the same as my tanks..I have 2 baby tanks running too!

I mentioned in an earlier post that Ph is not as important as water hardness in your aquarium. Below is an explanation I have taken from another forum, explained much better than I could manage. Useful to read I think.

You have to have a certain PH value
Fish are entirely unaffected by Ph within the range 5.5 to 9.0. In fact the range could well be wider than that, but filter bacteria efficiency reduces as the PH drops and the filter bacteria go dormant and start to die off before the fish start having any problems due to PH.
Fishkeepers who have considerable planting and CO2 injections systems often report changes in PH by over 1.0 within a few seconds of the CO injection system operating with no ill effects to the fish, thus proving that fish are able to cope with very large changes to PH several times every day.
Of considerably more importance to the fish is the Total Dissolved Solids (TDS) in your water. TDS is usually related to hardness and hardness is often indicated by PH, but they do not always have to correlate. Fish that are traditionally known as soft water low PH fish are in fact intolerant of high TDS, and conversely fish that are known as hard water high PH fish are actually intolerant of low TDS. Whilst it is possible to purchase a TDS meter, this is generally considered unnecessary as the PH and water hardness relationships are usually true, and therefore a safe estimation of TDS can be made from measuring PH, GH and KH.
The use of the PH up / down / all around type products is not recommended and can cause more problems that they attempt to solve. Whilst the Ph is unimportant, adding unnecessary chemicals to your tank is never a good idea.

Thanks. That's really useful.