Ok it's a blend of coco and soil, test the ph coming out after you water collect enough runoff.
I wouldn't call it coco. Ph of your tap is important but more so the alkalinity. Rather than get into all the details, the soil has the ability to alter ph to an extent. So testing the runoff will see where your at in a very vague way.
Because it's likely depleted of nutrients you can expect the ph of runoff to be higher and adding nutrients will help lower pH.... Enough that's a different story. Yes a ph meter would be helpful to you.
If you want to try understand what I'm talking about read here.
OK I'm going to do my best to explain PH since its something that is for the most part greatly misunderstood and can be confusing to new growers and even experienced growers alike. This will explain why we need both ppm and PH meters to give us informed information about PH This will be a...
www.thcfarmer.com
Imo picking up a bottle of ph down, some balanced nutrients and a ph and ppm pen should give you everything you need to get back on track. If going organic I'm not the guy to talk to and you don't really need the ppm meter. If going synthetic then those will give you a good chance of fixing this.