Help, I am baffled and ready to just give up on this tank. I have lost 5 platys in 2 weeks. There is one female platy and one female swordtail left in the tank. It's a 20 gal talk tank. 3 platys died after gulping for air at the top of the tank. One died from jumping out of tank ( tank is covered except for filter cutout). One platy got sucked up to the filter and died. Now, my swordtail and remaining platy are completely hiding and only coming up to eat. This is a 2 month old tank. It is cycled. Parameters are good according to master API kit. Ammonia 0, nitrites 0, nitrates 10 and pH 7.6. I have a hob aquatech 10/20 gal filter. Water temp was 78 deg but slowly reduced it to 76 hoping this would allow for more oxygen in tank. Added bubbler a week ago. The only two things I can think after researching and researching is I diffused essential oils in same room as tank. So today I added polishing pad to the filter hoping this may help reduce any water contamination, if there is any. Also, don't have a test to test gh thought maybe water is too soft for platys. Anyone have any other suggestions/ideas?? I have done several water changes using prime water conditioner. I don't want lose anymore fish! Sorry for the lengthy details. Thanks.