My plants have always been fine without any extra Co2, I don't know why that would suddenly change as nothing else in the tank has changed
No plants do well without CO2. Something has changed so less of it is produced in your tank. Any melting, holes in leaves, blackening of plants, falling leaves and subsequently algae is 98% CO2 related. When plants increase the biomass, there's more CO2 demand in the tank, so your tank currently is just not able to sustain those amounts. Also, when buying new plants depending on where you got them from, they are loaded with reserves and may do well for a few weeks to a month, then once that's used they start a sudden decline if the current tank isn't replacing these.