[Pink Flamingo melt and die off]

PH is about 6.8 and GH/TDS is about 600.

[Pink Flamingo melt and die off]

Sorry if this topic is covered somewhere else.  I am brand new to the forum.  I recently purchased my third Pink Flamingo Crypt from Aquarium Co-op, and for the third time it melted within a week and the roots and rhizome died off after about 3 weeks.  I planted the last one in front of a 75 G with a Fluval 3.0 over it at it's brightest setting.  The substrate is a dirted tank, capped off with gravel and the tank is about 3 years old, very established, but I do not use CO2.  I love the look of the plant, but I am pretty frustrated with my failure to be able to grow it.  I have had good success with most aquarium plants.  I was thinking my next try should be in a shallower tank, and planted in a small pot with a good aqua soil, such as ADA Amazonia.  Any suggestions are much appreciated.