Fat banana Royal Queen Seeds.

buy a 100pk of strips that resolve down to .5pH ... the ones broken down into 4 colors are easy to distinguish. this will 2-3+ years worth of spot checking for 5 USD. Probably cheaper than the meters and stock solutions to keep one callibrated... and you have to buy a new diode every few years anyway. strips are easier and cheaper in most contexts, unless you plan to use it every day (i.e. very frequently). .5 resolution is enough for gardening needs.

Your diary says the growing medium is Soil 22% then lists nothing else. What is the medium comprised of? it may need some CalMg, if its Coir or Peat it will benefit from this addition. BioBizz can work as low as 6.2PH so I don't think its a PH issue off the bat. One word of advice for using BioBizz nutrients. let them sit out for 12 hours once mixed up to allow the PH to drift. then adjust it if needed. Good Luck!

Not a pH problem, but the beginning of a cal/mag deficiency. Easily resolved by feeding with a calcium/magnesium booster fertilizer every week or two.