A 160 liter (~ 42 U.S. gallon) tank is too small for fish like Discus. I'd recommend an absolute minimum of 75 gallons (284 liters) for Discus. If you want to be really kind and understanding to these delicate and demanding fish, get something around at least 378 liters. Thats' what Discus need to be truly happy. Their tank needs perfect water quality as well.
The rest of the fish are fine for your tank.
If you tank was cycled before the problems began, it was most likely the medication that killed all the beneficial bacteria in your filter, and therefore caused another cycle. During a cycle, fish will die if they're exposed to
ammonia and
nitrite.
What are your ammonia, nitrite, and
nitrate readings? How exactly do you clean your filter and how often? Do you change any filter media? If so, how often?
If you have
ANY ammonia and/or nitrite in your water, you should be performing daily 50% water changes if you want to increase the chances of your fish surviving the cycle.
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