Hi, all. We have a 55gal tank that's been in operation for about 9-12 months now, and it seems fairly stable. Our pH is often low (6.4), but otherwise the tank seems fairly healthy at least to my untrained eyes. (0 ammonia, 0 nitrites, ~20ppm nitrates). The tank is filtered with two Second Nature Whisper Power Filters (I believe one is a 20, the other a 30) and has a couple of bubble wands and under-gravel bubble lines. The tank currently has 1 pleco, 4 cories, 8 swordtails (7 of which are adolescent), 2 platies, 4 rasboras and 3 zebra danios. The bottom of the tank has a number of plants, and a fair amount float on the surface as well. (I thin the floaters out when I change the water).
Lately, I've noticed that the platies and the mature swordtail will hang somewhat listless, fins drooping, near the surface of the tank. They may just sit, or they may have their mouths on the surface (looking to me like they're sucking in air bubbles). One mature swordtail died last week (unknown if it's related or not).
In the last couple of days, the pleco has started doing the same thing - floating in the floating plants at the surface of the water, upside down, motionless, only making ripples on the surface with his mouth. It looks like he too is sucking in air.
What does this mean for my tank? Are the fish not getting enough oxygen or something? Do I need better filters? Should I be changing out the water more frequently (currently about 1/4 water every 3 weeks or so) or changing the filter charcoal more frequently (done very infrequently, I confess!)
Any suggestions would be welcomed. Thank you!
EDIT: Here's a couple of pictures (click for larger versions). You can see the pleco at the top of the tank hanging upside down in the first picture (near the middle). At the time of this picture, he'd been there motionless for about an hour.
Lately, I've noticed that the platies and the mature swordtail will hang somewhat listless, fins drooping, near the surface of the tank. They may just sit, or they may have their mouths on the surface (looking to me like they're sucking in air bubbles). One mature swordtail died last week (unknown if it's related or not).
In the last couple of days, the pleco has started doing the same thing - floating in the floating plants at the surface of the water, upside down, motionless, only making ripples on the surface with his mouth. It looks like he too is sucking in air.
What does this mean for my tank? Are the fish not getting enough oxygen or something? Do I need better filters? Should I be changing out the water more frequently (currently about 1/4 water every 3 weeks or so) or changing the filter charcoal more frequently (done very infrequently, I confess!)
Any suggestions would be welcomed. Thank you!
EDIT: Here's a couple of pictures (click for larger versions). You can see the pleco at the top of the tank hanging upside down in the first picture (near the middle). At the time of this picture, he'd been there motionless for about an hour.