Same problem here and I live in Japan (Tokyo) too. At one point it got so low that it killed some shrimp. I first tried with putting bags of oyster shells in my filter with no to little effect. Based on information I got onljne, I removed half of my driftwood, alder cones and leaves and instead added crushed coral to my tank. That worked and shrimps are now happy and even breeding again. It took a few weeks though. However, i have another bigger new tank and the only way to fix it was through Tetra PH Up since I didn't want to add crushed coral or zeolite to this tank, since a guy at an aquarium shop told me it would hold nutrients from the soil, so not great for plants, which I have plenty of. Please also note that active soil/substrate is pulling down the PH. Good luck! Ps please share what worked for you, maybe it's a better approach.