Hey, welcome to the wonderful hobby of fish keeping! I can definitely agree with the above posts on reviving weaker fish, or possibly maxing out you bio load. But I have one additional point to add, get fish which are suited for your water, I personally have 7.8-8.0 ph well water, and every soft water fish/ shrimp I have added to my tank have died in a relatively short period of time. So it’s always a good rule of thumb to get fish suited to your water. This can be done by asking your lfs the water parameter they keep a certain fish in, or look to google for the help. Happy fish keeping, I hope this helps