I have a 27g tank with a whisper filter/ temp is 28C/ gravel. pH~7. Did water change (less than 25% and agitated substrate yesterday and put in 2 new plants (onion plants). Also installed a DIY CO2 pump for plant growth. All done yesterday at 7pm. Now 2 of my 3 danios are not swimming very well; one keeps on getting sucked onto the filter. He is listless and just lets the current carry him along. When I bought the CO2 pump the guy told me that if my fish are at the surface gasping to turn up the current. I did, and have taken a little water out so to break the surface tension (so the water falls further). I have 2 gouramis and 6 rasboras, they are all fine still, as a matter of fact, they are great. Is it the CO2? I know that the rasboras stay close to the top of the tank usually and that gouramis are used to living in low O2 conditions. The other thing is that I started with 6 danios and they died off, however, I thought that it was because my 2 pea puffers ate them. The remaining danios have missing tails. I got rid of my puffers 4 days ago. So badly bred fish or poor conditions? Likely that they were eaten or died on their own? I also started with 4 ottos and am down to 1. I thought that my rasboras were the more finicky fish in my tank so I don't understand. Thanks for helping!