Know your water ph before adjusting it. If necessary, you'll need one or the other...not both. But, I use Phosphoric acid for ph down. (phDown by General Hydroponics) One gallon lasts about two years, and is safe to use. (the ladies love the stuff)

Take a sample of your water in a clean jar to an aquarium store, and ask them to test the ph. (or buy one of the freshwater aquarium ph test kits for under $10)

The plants are hungry. The nutes in pre-ferted soil mixes don't last all that long, especially in seedling mixes. What are you using for nutes? Whatever it is...go half strength for first couple of feedings, increasing as plants get larger.

The larger plants will need a better foothold as they grow, and the seedling mix doesn't offer enough support to a larger plant. (too fine of a medium) When you next transplant...don't use the seedling mix.