If someone handed me a grow op and took off without telling me anything about it but the age I would not start by adding nutrients to it and flushing out everything once I saw a toxicity. I would've just watered until some deficiencies showed up then gone in with half the strength of the bottle directions for nutrients. If you're going to flush watch the runoff since you don't have the ppm meter and feed half strength if not quarter strength after the flush and see how it reacts before proceeding with more nutrients. I had some seedlings react poorly to nutrients at 1/2tsp to a gallon recently so I too have to be careful with my nutes.