Firstly there is a mix of hard and soft water fish so as others suggested to me find out the Gh and stock from there , I wouldnāt add anything to the tank sounds like the bio load is high as is but Iām no expert I rely on information from here way more than Iād admit lolYeah that's why I came here. I wasnt sure if it was over stocked as tetras have a very small bioload. I was gonna suggest chili rasboras if she really wanted to replace them but idk.