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I've got a 64 litre tank heavily stocked with 15 blackskirt/black neon/glowlight tetras and an Apisto. Filtration is provided by an internal power filter so the only method of getting oxygen into the tank is an air stone or surface disturbance. I have been using a ceramic diffuser to distribute one 2l diy co2 bottle and have a kh of 6 - my ph drops to 6.6 from 7.2 which gives me a co2 level of about 45ppm. I've noticed changes in behaviour of the fish, the apisto doesn't seem phased at all which is strange as I read larger fish show signs of co2 toxicity faster than smaller fish. My tetras on the other hand seem to get lethargic, or act strangely sporadicaly jerking and darting around, the black skirts seem to kind of bob in a 45 degree head up angle. Is this a sign of toxicity? I was concerned about low oxygen levels so I ran an airstone over night and by 10am it had completely gassed off the co2. i was hoping it would maybe lower it slightly. I have angled the output of the filter to ripple the surface as much as possible without breaking the surface. The fish aren't gasping at the surface or anything but I have definately noticed a change in behaviour. Being heavily stocked, is there a way of maximising oxygen without gassing off co2? Or is it a case of lowering the fish load / ditching the co2?

here is a pic of the tank - do you think this amount of plants should produce enough oxygen for the fish?
 

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A ph decrease of 0.6 means the ppm of CO2 went up by a factor of about 4, so you probably have around 10-15 ppm of CO2. That will only harm the fish if your water is extremely low on dissolved oxygen, which I doubt it being. It takes a lot of attention to detail to both boost the dissolved oxygen and maintain the high dissolved CO2. One method would be using a wet/dry filter while also avoiding any splashing of the returning water, and even at least partially sealing the filter from the atmosphere.
 

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Thanks for the reply - could there be a case for the micro bubbles irritating the fish? I assume not given the amount of people using diffusers... Also I assume the whole point is healthy plants boosting the oxygen levels themselves. All I know for sure is my fish definately act different with the co2 :/

Also I made a mistake earlier - my normal ph is 7.4 - well it shows 7.4 on the high range test and 7.6 on the low range. I just checked again and ph is now 6.8 after about 9 hours of co2. I have real trouble telling the difference in the colours down to 6.4 on the API chart. Is an electronic ph checker worth it?

Maybe a classic case of over worrying - fish all seem fine today and I notice the 45 degree bobbing action of the black skirt tetras is mainly down to them "hovering" facing into the flow of water. So I guess unless the fish are at the surface gassing everything is ok?
 

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I have overdosed my fish with CO2 too many times to admit. Every time I did it, the tetras, and similar fish all clustered in a top corner of the tank, usually a corner where I would expect the CO2 concentration to be the lowest. Other fish would lay on the substrate, at times laying sideways, and some would have badly faded colors. To me those are absolute indications of too much CO2, with too little oxygen in the water to compensate.

When I first started using CO2 I noticed a lot of stress in the fish at what I soon learned was barely over 3 ppm of CO2. Eventually I realized that the only stress was what I was feeling from fear of harming the fish. Once I started using a drop checker I found that I was able to stifle my fear and increase the bubble rate enough to get significant CO2 in the water. For me that was the primary reason I used a drop checker, something I recommend to anyone who has limited experience with CO2.
 
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