snazy said:
Edit: The crypt was bought from someone who didn't inject Co2 into their tank either and just had sand substrate and LED lights...
That doesn't mean a thing. Has any of you ever measured the CO2 levels in the tanks(which is kind of impossible)? They could be totally different. Your plants are telling you they aren't getting enough co2, maybe even not getting enough flow/distribution of CO2 if you've changed a filter or the filter position. Honestly, it's not some kind of mystery no one has experienced.
Filter hasn't changed, I've had the Crypt over a year with no added Co2 and it's been in this tank since last year with not problems until last week which was before your plants were added it suddenly started melting. I know it's lack of Co2, I'm not saying it's not. I'm more trying to figure out why it's lack of Co2 when nothing had changed when it started melting.