I have a 20 gal freshwater aquarium. I can't seem to keep my fish alive, other than 2 white clouds. My PH level is very high. I have used PH- drops for a month now and it is still high. I asked my aquarium shop and they told me to take the sea shells out of it, that they will raise the PH. They have been out for over 3 weeks, I have changed 3/4 of the water (the new water tested low before adding). I do have snails in the tank, everything else is plastic. What can be causing the PH level to stay so high? I would really like to get more fish, but can't until the PH level is corrected. I have 1 waterfall filter, 2 tube aerators and a long wand aerator. Please help!