Loaches Online Forum

I was just wondering how one can tell if there is sufficient oxygen levels for the loaches in a tank? The obvious sign is when loaches are going up to breathe or gasping for air, but that is a bit too late.

Are there other signs to look out for or to measure? I know that raising the temp to 30oC and above creates oxygen problems and that dropping the water level helps, but what are the key drivers of determining oxygen levels in a tank? Is it mainly due to filtration, temperature, water parameters and tank stocking?

I haven't noticed any issues with loaches in my tank, but I was just curious.

There are no easy ways to know the true oxygen level unless you have an oxygen meter. These meters are expensive, so the best way to avoid oxygen depletion is to avoid certain things.

Firstly, avoid overstocking the fish tank. This is where I hate that myth called the “inch per gallon rule”. When it comes to oxygen, it's the foot print of the tank that matters most, not the volume. Long shallow fish tanks are better than tall tanks with a smaller foot print because the long shallow tank will have much more surface area per volume for oxygen exchange.
Plenty of surface area, water movement, and surface agitation help ensure that O2 levels are sufficient.
I don't know of any formula to determine how much surface area is needed for each fish. It would probably be a great formula for someone to come up with. Inch of fish gills per Sq foot of surface area?

Free Floating Bacteria use up oxygen and thrive on suspended solids (fish waste). These solids are usually naked to the eye in an aquariums because filters will keep breaking apart the solids, dividing them up into smaller pieces until they are nearly liquefied. In this case you end up with a slurry of fish waste products and water. This slurry is even better for bacteria to thrive on because the smaller those suspended solids get, the more total area their will be for bacteria to grow on. To avoid the potential bacterial infections and oxygen problems, clean the filters, and do water changes. Canister filters are very good in this instant, and HOB filters not so good. This is where an UV sterilizer can help improve water quality.

Nitrifying Bacteria use up oxygen also. The more ammonia/nitrite that needs to be processed, the more oxygen these bacteria will use up. This is another reason why it's important to keep the filter clean. A clogged up filter could quickly become oxygen deprived and ammonia/nitrite will build up inside killing off the nitrifying bacteria. Bio-Wheel filters are very good in this instant, and canister filters not so good. It's important to have redundancy with either filter because you never know when one might quit working. Trickle Filter Sumps are the champ when it comes to the best all around filter.

Plants and Algae use up oxygen at night and produce CO2. This is potentially dangerous to fish. CO2 is an anesthetic and if the levels get high enough it will put the fish into a lost state of consciousness or even kill the fish. The lack of oxygen can kill the fish also.

Temperature, like you mentioned, matters also. The higher the temperature, the lower the oxygen levels will be.

Martin wrote an excellent article on Oxygen a while back.
http://www.loaches.com/articles/oxygen-in-the-aquarium


I can't think of anything else at the moment that's oxygen related.

I have seen only one formula relating stocking levels to surface area. It applies only to small fish, about 2" max.

Measure your tank Length x Width.
Make one measurement in inches, the other in feet. (Weird, but true) When you multiply these two figures you get the number of fish-inches that might be supported in a tropical, fresh water tank with reasonable water circulation.

If you want to translate that to metric:
Measure L x W in cm.
Multiply. Divide by 30.
This is the number of cm of fish you might be able to keep. It relates only to fish of about 5 cm or less.

A couple of test cases:
A tank 1' x 3' can be measured out as 12 x 3 or as 1 x 36. Either way, 36" of small fish is the max.
Same tank, metric: 30cm x 90cm = 2700 sq. cm. Now divide by 30 = 90 cm of fish.

A tank that is 18" x 46" could be measured as 1.5 x 46= 69" of fish.
In metric: 45cm x 115cm / 30= 172.5 cm of fish.

Oxygen needs as well as waste production is related to the mass of the fish, not its length. If you think of the volume of the fish as length x width x height this gets you pretty close to mass.

A fish that is the same shape as a smaller fish but twice as long produces 8 times the waste and requires 8 times the oxygen. A similar fish that is 3 times as long as the small one produces 27 times the waste, and requires 27 times the oxygen.
The formulas that work for fish that are around 1-2" absolutely fail when the fish grow larger.
A fish that is 1" long uses "one fish-mass" of oxygen.
A fish that is 2" long uses "eight fish-masses" of oxygen.
A fish that is 3" long uses "twenty seven fish-masses" of oxygen.

If you want to cycle with fish (boo! hiss!) and start with 8 Zebra Danios (They can handle having their gills burned by ammonia better than some other fish) You could trade those in on ONE two inch fish.
If you started with 27 Zebra Danios you could trade them in on ONE three inch fish.

Much better to use the Fishless Cycle.