Single unit waterfarm nutrient/top-up question

Hi guys this is my first post......Let me start off by saying...THanks for all the great info you guys are sharing. This is my first grow(god bud) and things are going well. I have been topping up the farm with h20 only and changing the res every 4 days throughout the grow.... The girls have just begun their second week of flower...they are getting big.... really big...and they are very thirsty. |It only takes about 36 hrs for the top -upe to equal the 2.5 gal res size. So should I change out everything at this point or continue to fill and use a 4 day schedule like before? I don't have an ec/ppm meter so I cannot top up safely with nutes? Or can I?
Thanks in advance;]!

You really do need to get a PPM and a PH meter. Without a PPM meter, you have no idea if you are feeding the plant enough or too much. I think it is awesome that you have gone this far without one. You can always mix up extra solution when you change out tanks and top with it, but if plant is taking water and not nutes, you risk burning them. An ounce of good bud can fetch up to $400. A good PPM meter is about $60..... Just some food for thought.

Thanks....that's a good point....I have been able to succeed to this point i think beacause of the small res and complete nutrient/water changes......up until now.....I will have to pick one up. Thanks for the quick reply!
Peace and happy growing