Well, when you see pH problems, ask yourself,
-Is the water flow high enough throughout the system?
-Is there any material that may be leeching into my system?
-The media being used, is it safe?
Try adding a buffer before adding an acid like muriac.
The fish may be your solution. Tap water at a pH of 8.4 sounds strange. What kind of test kit are you using? The dip sticks or red litmus paper are the best solutions i think.
If your tap water really is 8.4, try buying a few gallons of distilled water, i dont know how large your tank is, but it doesnt sound like a tap water problem.
You could try running the water through aquarium activated carbon.
It doesnt sound like a water problem, but some other component of the system that's screwing up the water.
Actually, some parts of the country, once you let the dissolved carbon dioxide escape from the water, a pH of 8.4 isn't that crazy. If I test my well water right out of the faucet I'll get a reading between 7-7.4 usually but if I let that water bubble overnight and then test again, 8.0-8.2 is normal. Some places have liquid limestone coming out of the faucet and 8.4 is normal and the reason some parts of the country have lots of water softeners and RO filters for the houses.
If pH is high, adding a buffer isn't going to help.
But I agree that using acid is not a good long term solution either. I admit I've had to resort to it myself when I don't have any rain water though. Problem with using acid regularly is it will simply mobilize more of the calcium into the water and likely cause potassium lockout problems.
Anyone using SeaChem products?
the wood we use is DRIFTWOOD FROM THE LAKES OR CREEKS/RIVERS ETC. It is smooth and dry and light. Probably sycamore? It has worked wonderfully. Don't use Pine good luck
I don't have numbers because it depends on so many things as to how much rain water it would take. Do you know how much water you top the system up with? I know that if I use rain water to top up my 300 gallon system for a month the pH definitely drops compared to if I'm topping up with my well water.
Now I know of people who say they have done things like add a couple oak logs (like pieces of fire wood) into their tanks to help lower pH. I'm testing that idea out on one of my systems right now but It hasn't been a week yet so I'm not seeing any results.
A water softener that removes minerals from the water (like for keeping lime scale from building up in your water heater and such) can work but it will likely also add some salt to your water so it may not be that great.
RO filters can be costly which is why I haven't gone that way myself but I'm having issues with lack of rain too.