Grand black & purple Dick

Depends on how the ppm and ph have changed from the original level. If ppm has dropped and ph risen slightly I would add the full line up to get it back to a slightly higher ppm than previously set. If ppm has risen slightly and ph dropped I would add just water to get it back to a little lower ppm than previously set. If the ppm and ph are pretty much the same I would just top it back up with the same ppm/ph level as previously done. If the ppm has raised a lot and ph has dropped by a big margin I would empty the tank and put in a fresh mix as it would obviously mean the plants were just taking water and not the nutes, this is usually due to too higher ppm, tank is overdue a change or nute lockout.