I stock up on consumables and back up hardware.
I prefer dosing pumps over a calcium reactor, so once a year I buy the BRS five gallon KH, CA, MG on sale.
Really helps match my IO batches to save on more expensive salt.
I also splurge and buy the five gallon GFO bucket.
Enough RO filters, blocks and DI, super glue, testing kits to get through the next Thanksgiving sale.
I always have a brand new back up return pump, media pump, and doser just in case of a failure.
Be proactive VS reactive. The little amount of money you save by planning helps make this hobby more affordable for the long term.