I bought a 10 gallon aquarium and bought a pair of fishes like platies, mollies, rainbow shark, tiger bard and one snail and algea eater. I didn't know about the cycle of water and put all fishes in water. In few days almost all my fishes died. Now i have left is a pair of rainbow shark, one snail that look fine and one molly and tiger bard that look sick (ich).
Although most of my fishes have died but i have completed the water cycle. Now i want to put a few new fishes back but i am confused that is my water OK. I have completed the cycle process due to which no water is save for fishes but those fishes that were sick and died may have polluted the water with diseases.
Please tell me what should I do?
Should I change the water or not?
And if I change water how it will again be toxic for fishes then what should I do?
Please suggest me anything that you think can be helpful to me as I am a starter?

Medications that I used are anti-chlorine and methylene blue.