Get a pH tester and a TDS (Total Dissolved Solids) or EC (Electrical Conductivity) meter, if you are serious about growing a hydroponic garden. Until then, using a seaweed-based fertilizer called Maxsea will solve your problem. The first ten days your plants have roots, feed them 400 ppm Maxsea 3-20-20. This is about 1 level teaspoon/gallon of tap water. Add to this 1/4 teaspoon Epsom salts for magnesium.
Feed them 600 ppm Maxsea 16-16-16, after the seedling stage. This is about 1 rounded teaspoon. Add 1/4 teaspoon Epsom salts to each gallon. You can feed most plants in this way through the vegetative period.
Feed your garden a rounded 1/2 tsp Maxsea 16-16-16 plus a rounded 1/2 tsp Maxsea 3-20-20 in each gallon of water. Be especially sure to add 1/4 tsp Epsom salts to each gallon through the flowering stage. You could switch to straight Maxsea 3-20-20 at this point, but most plants look a little nitrogen deficient by the end. Feed your garden in this way for two weeks, while the plants transition from vegetative growth to flowering.
Feed your garden 800 ppm Maxsea 3-20-20 until you near the end of flowering, after the transitioning period,. This is about 2 level teaspoons per gallon. Make sure to add 1/4 tsp Epsom salts to each gallon. The very last 10 to 14 days before you harvest, you will want to feed your garden only plain water. Flushing your garden will remove excess nutrient salts from the plant tissue and will improve the aroma and flavor of your garden produce.
Plants use some nutrients faster than others. After two weeks of using the same nutrient solution, the solution will no longer have a good balance. To avoid nutrition problems, empty the old solution every two weeks and start with fresh water and nutrients.
Seaweed additives are a popular choice when it comes to trace nutrients and plant hormone supplements. Since Maxsea fertilizers are seaweed based fertilizers, these benefits are built in and no additional supplements are required for these two things.
By carefully measuring the fertilizer when you mix up the nutrient solution, you can get away without the initial cost of an expensive TDS meter or EC meter as well. The best advice here would be to always top off your nutrient reservoir with 1/2 strength solution whenever it is a little low. Every two weeks, start over with fresh water and nutrients to avoid a nutrient imbalance in your solution.
The completeness of Maxsea offers you a very simple feeding plan. Between the two formulas (16-16-16 and 3-20-20) it is easy to provide a good balance of N-P-K for any stage of growth. Even so, if you carefully read the fertilizer ingredients, you will find Maxsea is missing one thing... magnesium. By adding 1/4 teaspoon Epsom salts/gallon, you inexpensively avoid having any problems with a magnesium deficiency.
When you mix up Maxsea in a nutrient solution, it has a nearly perfect pH. What's more, the pH is stable in solution for many days. This is great for the beginning hydroponics gardener. This means you can get away without the initial cost of an expensive pH meter in the beginning.
Replenish the water the plants are using in between nutrient changes. The safest strategy is to top off the nutrient reservoir with half strength nutrient solution.