I have been having a similar problem. Can you get your lfs to check your KH, carbonate hardness, from the tank and from the tap. Or if you have the test kit you can do it of course. Also would be good to know your GH, general hardness.
This situation is really increased when you do a lot of top offs and not pwc each week, or fairly small pwc. Then there is a low level of KH and GH just keeps increasing and then there isn't enough to buffer. Sorry I am not the specific scientist type so you will need to study about this more, or someone can add a link for you to better specific info.
My KH from the tap basically won't register on the test kit. So it makes the tank unstable. There isn't enough in there to be a buffering agent for the pH. Lack of healthy size pwc, personally would recommend 1/3rd to start.
I have just been working on this problem.
It is a bad place for the fish and plants.
I used API pH increaser and it didn't really help.
So now I have been using baking SODA, and Calcium additive, Kent brand usually used in SW.
I am doing this gradually and so far I have a pH which is getting closer to the tap number, but KH for me is still low, so continuing to add Calcium about every 3-5 days.
Last week I did a massive substrate clean to get rid of gunk in the Eco Complete and that went great, and now this week I am gonna do the clean out on the canister filter and add 3 bags of crushed coral. The tank is 72G.
Didn't want to do them too close together. My filter BB crashed when this started so I am about at the end of building it back up and having the tank cycled again.
This was a major pain, changed a million gallons of water, lost 2/3rds of my fish and plants. Most people I talk to say they have crushed coral in their filters here where I live. Previously I kept Cuttlebone in the tanks to feed the snails, but these 3 tanks I had the major crashes with didn't have the Cuttlebone anymore because it had been dissolved over time.
Good luck, hope that helps you in some way.
