First, do not try to keep it at 5.8 or at any exact point, just try to keep it between 5.5 and 6.5, and they will do fine. Adjust it once daily.
Come home, test it, and find (for example) that it is 6.9, too high. Follow the instructions on your pH Down, remove onequart to one half gallon of water and add it back with pH adjsuted water. If you removed a half gallon, make a new half gallon read 5.2 to 5.5, and add it to the tank, and then the tank will be close to 6.2 to 6.5, and that is fine. You can wait 4 to 8 hours and do it again, but do not adjust it a lot at one time.
Never add the UP or Down directly to the tank. You could really mess up doing that, that way.
Start at Post # 38, page 4 here and read up on pH:
Roseman's DIY Bubbleponics-DWC Tutorial
you'll be a pH expert then.