Remove the pleco and i wouldnt say you are overcrowded. Add the pleco into the mix and it depends on what kind of pleco it is. A common pleco for instance can get 2 feet long and will outgrow your tank.
Do you know what type of pleco you have?
Your plants are probably just transitioning from emersed to submerged growth. They will have been cultivated partially out of water with easy access to CO2
from the atmoshere. Now they are underwater that CO2
is cut off and the leaf structure needs to adjust to its new environment. It does this by growing new leafs more tolerant of being underwater, and old growth may die off completely. It doesnt look like the plants you have are high demand, and a sand substrate will be fine. You may want to dose some all in one liquid fertiliser and add some root tabs into the substrate.