60L just wont cut it for discus.
if you do have a chemical problem in the tank, the low volume of water will mean the change in water parameters will be more dramatic. try putting them in a larger tank, or maybe planting the tank as much as you can. Indian fern is great - you can leave it on the water, and the roots get rid of a lot of nasties.
as for ammonia spikes - more frequent water changes will be needed.
hOPE this helps, and the rest of your fish do well!