Is it necessary to have a second airstone when having sponge filter?


So I have two tanks.

One 10 gallon, and one 40 gallon.

In the 10 gallon tank, my Hydro sponge II filter is the only filter. I use an airstone in it.

Is it necessary to have another airstone to pump air in this 10 gallon tank? Or the airstone inside the sponge filter does the same thing as a regular airstone while working for the sponge filter? (the airstone in the sponge filter is much closer to the water surface than the stand alone airstone which are usually at the bottom of the tank)

I bought a single Rena 300. I thought it would be quiet from all the reviews but it is not THAT quiet. Actually it is pretty loud when I turn the airflow to near max. It has nothing to do what's around it or below it, as the noisy won't decrease at all when I hold the air pump in my hand.

On the top of that, I found the single air outlet can't do the job for both tanks. I have a full package of all kinds of valves so I divided the air lines.

The Hydro Sponge IV in the 40 gallon tank isn't getting much air and a second air stone at the bottom of the 40 gallon tank gets 0 air flow, while the Hydro Sponge II in the 10 gallon has too much air flow.

I guess the air is always looking for "easy" way out. Since the airstone in the Hydro II (in 10 gallon tank) is closer to the water surface than the airstone inside the Hydro IV(in 40 gallon tank).

Any solutions? Or do I have to get a second air pump?

By the way, gallon per hour for sponge filter really is depended on the air pump output right?

One more question. The sound of air bubbles explode at the water surface is making lots of noises. More so than the air pump. The water also spilled onto the top cover from exploding air bubbles. Are there any good solutions to them(noise and water everywhere)?


Good morning.

If splitting the line does not work, then you will need another air pump I'm afraid. I too have a sponge filter and agree the noise of the bubbles bursting is noisy, but this tank is not affecting anyone trying to sleep as it is in a living room rather than a bedroom!

I have found when splitting the airline that one side generally gets more air than the other, irrespective of where the outlet is, as I used this for a regular old airstone, and one side had more bubbles than the other.

Gallon per hour filtration will depend on how high the air pump is set. I have a valve and it can regulate the air flow, mine is set to about half as the filter is for a much larger tank than it is in at the moment.

Finally, you shouldn't need a separate airstone in your tank as the filter will be producing enough surface movement to aerate your tank.

Hope I helped a little. If I'm wrong, then someone will always be kind enough to correct me, so wait for a few more responses before finalising your decision - I would hate to know I gave inadequate advice!


I bought a little valve thing that lets you control how much air goes to each tube by means of a knob. It'll close off air to one side so the other gets air and then you adjust it by turning the knob. Cheap little thing at Petsmart and I use(d) it on two different setups.

As for the airstone, it might not be necessary but all my fish love to surf the bubbles so it's a source of fun for them. They all get head down into the bubbles and try to swim against them. It's really cute. The fish in your 40 gallon would be able to enjoy it more because there's more vertical space to surf.

Air pumps and bubbles make noise, it's just a fact of life. They're a great source of white noise and I actually like hearing the hum and pops when it's quiet around here. The sounds will quickly fade into the background the longer you have them.



I run many 10's and 20's with sponge filters only. As long as there is sufficient surface agitation, you won't need another airstone unless you want to add it.


I really thought the oxygen from the airstone dissolve in the water during on their way up to the surface.


I really thought the oxygen from the airstone dissolve in the water during on their way up to the surface.

That's a common misconception. It's actually the surfae agitation that helps oxygenate the water.



To add to Lisa's post.

The surface agitation facilitates the gas exchange process by allowing the carbon dioxide to leave the water and be exchanged for oxygen.

Whilst surface agitation is one of the best forms of gas exchange, any process that moves the water in contact with it's surrounding atmosphere is helping the process. HOB filters, wet/dry filters etc also facilitate the process.

The bubbles produced by airpumps don't actually enter the water by just being in the water. It is when they burst at the surface that the gas exchange takes place. The advantage with air bubbles is that they burst at the water's surface, meaning that the oxygen is closer, thus they help add oxygen by over-supplying the surface area.

The bursting of the bubbles also creates minI agitations of the surface, hence they help increase the oxygen in that way too.

Sorry if that was too much "science", but hope that kind of explains it all.

edit: Note that this only applies oxygen. Carbon dioxide (CO2) gas actually 'dissolves' into the water the longer the bubble of CO2 is in the water.

A good example of all this, if you blow air into water, the bubbles just rise straight out. But in the case of Sodapop (softdrink) the carbonate gases stay 'dissolved' in the drink. Once you open the bottle of soda and start drinking it etc, the movement of the drink expells the carbonate gas (CO2) and the drink goes flat. In other words, the gas exchange of CO2 being exchanged for oxygen, the same as in the aquarium (kinda)

Sorry, I know there was no question about CO2, but I thought it was a good example.

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