(the new water tested low before adding)
Before adding what?
A few things could be causing the high Ph.
One could be your source of water. Try testing your tap water for readings on Hardness (GH), Alkalinity (KH), and Ph.
Place some tap water in a glass, wait 15 to 30 minutes, then test and let me know the readings.
Another cause could be the chemicals your adding to the water. Some water conditioners buff the water and raise the Ph and some are not compatible with water tests which cause inaccurate readings.
What chemicals (other than Ph down) are you using and what water test are you using?
Another could be the type of substrate(gravel) and décor your using.
Crushed coral (which sometimes looks like regular gravel or sand ) buffs the water causing harder more alkaline water which will raise the Ph.
Shells will definitely buff the water also, but you removed them, so that's good. There could be traces of the minerals left from the shells still dissolved in the remaining water that has not been removed yet, but with regular sea shells, you should have noticed a lower Ph after the water change...
So lets look further and see if we can figure out what's causing it.
For now, lets not use the Ph down.
If there is something in your water or tank causing the spike, the Ph down will only lower the Ph for a short amount of time. What ever is causing the water to have a high Ph will just Buff the Ph back up.
This rise and fall of the Ph is deadly to fish.
Could I get an aquarium reading of:
And a tap water reading of:
Any thing you've added to the tanks water
And what kind of substrate (gravel) your using.