fish lover wrote:

Anyone knows a good PH monitor? I don't like the strips and don't want
to pay too much.

The cheapest option probably are the pen-shaped portable ones, where the
electrode and the instrument form a single, small unit. Precission is
limited, but more than sufficient for aquaristic purposes. Lab supply
companies sell them for about 50-70 US$. Only disadvantage: they run on
button-cells, which don't last long and are quite expensive.

Remember that you need to keep the electrode in storage solution when
not in use. Never allow it to dry out, as it would become busted. You
also need calibration buffers (usually pH 4 and pH 7).