I have 5 Cory catfish and one betta. I know that betta can survive in low oxygen situations but some people recommend getting a air pump for car fish is it needed? I have a lot of plants and a aqua clear 30 which creates oxygen.
I always use aeration in my tanks. It can help oxygen levels with no negatives. And they're not very expensive. Although cories can come to the surface to gulp air, they will probably do so less often if you had an airstone running.
If you have cories , you might enter our Fish of the Month contest. This month we are featuring catfish only.
Historically air stones have been thought to be beneficial in the aquarium with the O2/CO2 gas exchange as bubbles break the surface. However, this gas equilibrium does effectively reduce CO2 in the water as O2 increases which may negatively affect plant growth.
In a high tech planted tank using compressed CO2 delivery, an air stone would be discouraged since it somewhat defeats the purpose of the CO2 additive.
However, in other tanks, planted or not, the benefit may outweigh any negatives.
I put an air stone into my tank in summer when it was very hot to ensure enough oxygen. The tank has low flow because, like your betta, most of my fish don't like much current.
My pepper Cory's spend so much time "playing" in the bubbles I have left it there for them as this creates a very localised area of high flow which the Corys enjoy. All the rest of the fish just keep out of it.
The gas exchange that occurs primarily at the surface is what you must ensure is adequate. This is especially important in planted tanks, and I have personal observation of this. My tanks are relatively well planted, and this means that during daylight (if intense enough for the plant species) photosynthesis is occurring which is using CO2 and releasing oxygen into the water. During darkness (night), this ceases, and plants respire continually just as fish and many types of bacteria are doing, and this uses oxygen and releases CO2. CO2 is also released through the breakdown in organics in the substrate. At night CO2 levels will rise; this occurs in natural waters too, but in the close confines of a planted aquarium this can be very significant. Ensuring the filter is providing adequate surface disturbance to allow oxygen in and CO2 out is important for all fish.
Some years ago I noticed that my cories had faster respiration in the early morning. I increased the surface disturbance provided by the canister filter return spray bar so it rippled the surface at that end of the tank. Problem solved. Ever since, the respiration rate of the cories has been normal when the light first comes on.
Surface disturbance is the key issue, and airstones are not going to provide much of this by comparison.
The issue of driving off CO2 is debatable. I used to accept this, and it was general advice, to keep surface disturbance (and air stones) minimal. Seems that may not have been accurate advice, as many planted tank sources are now advocating quite the reverse. Given my own observations, where the plant growth has not been affected by the increase in surface disturbance but the fish have been helped...I would not worry much. Provided of course one is not creating a raging rapids but just ensuring decent surface disturbance.
Another case in point.... As I have reported, I have high nitrates in my well water. Recently though, the water from the well is only about 5ppm in nitrates and filtering is a pain, so I thought I'd do a water change from the 'tap'. Shortly after, my fish were gasping at the surface. I wondered if there might be excessive dissolved CO2 in the water...so in another experiment, I filled a 45g Rubbermade Brute can and ran an air stone with fairly aggressive bubbling for 24 hours - problem solved!