Ivanrad

Just use RO water, then you can add whats necessary for your FW fish.

i hadn't finished helping you. It looks like your water pH is a little high for cardinal tetras. your tank is TOO small for the shark, and also your parameters weren't good for him. as far as i can tell, roseline sharks are actually denisons barb.

i would not replace either of those fish. instead look up fish that would do well in your water. also, invest in that liquid test kit, it'll help you a lot.