Hey guys I need some help. I am fairly new to the aquarium hobby. This is my first tank where I new about the nitrogen cycle. I have a 29 gallon tank with only an aqua-tech 20/40 HOB filter that came with the tank kit. I know I need to replace it but it has been doing a good job for me. The fish that I'm having problems with are the harlequin rasboras. I had 3 zebra danios 3 harlequin rasboras 2 otto cats and 5 shrimps then I got 4 more harlequin rasboras. Of the last rasboras that I got 3 of them died within a 4 day period. All the other fish are fine so I don't know what is going on. I tested my tank water and it is perfect. I have 0 ammonia 0 nitrite and 15 to 20 nitrates and ph 7.4. I do partial water changes once every 2 weeks. Any help and advice would be appreciated. I don't know I might just have gotten a bad batch of fish.