Looks like ph is out to me, if you aren't adjusting the water or nutes, then it's almost definitely that, tap water tends to be around 7-8, depending where your water comes from. You want it between 6.2-6.8 in soil. If you can't afford a proper meter, get an aquarium test kit, it's a little bottle of indicator solution and costs a couple of quid, any good pet shop should be able to sell you it. It's not ideal but should give you a rough idea of how much to adjust your feeds. Ph test the run off after you water as well, to see if there's a difference between what you're giving them and what comes out the bottom.
I don't see why anyone would spend money on Ph up/down when almost everyone has the stuff in their kitchen anyway! Lemon juice or white vinegar work fine for Ph down, just add a drop at a time then re-test until you figure out how much you need to be adding. Baking soda does Ph up, which is rarely required unless you have very acidic nutes.