chagovatoloco

Ok so I have been reading and over thinking as usual, and I want to know what the truth is. In a tank with no co2 added dose a hob filter add co2 in to the water column or dose it dissipate the natural co2? Basically is a hob filter and asset or a deffest in a tank where no co2 is added.

PlantedTank

I think there are two camps on this subject and they say opposite things, so it's probably difficult to get one single answer.

My opinion is that the CO2 pressure in the tank will be greater than the atmospheric pressure during the nighttime because of biological respiration. So, at that time, to a point, the more surface agitation there is the more CO2 will be lost from the water. Surface agitation basically expands the surface area of the water surface so more gas exchange can occur, for example, the surface agitation of water coming out of a HOB.

During the day, in a planted aquarium, the plants will take up available CO2 when light and nutrients are sufficient. Where no additional CO2 is added, the tank CO2 pressure will fall below atmospheric pressure and gas exchange will work to add CO2 from the atmosphere. In this case, an increased surface area would help CO2 enter the water from the atmosphere. It might marginally help plants photosynthesize longer, but imo not significantly. So I would say that overall, the more surface agitation there is, the more CO2 is lost, and in those terms, a HOB producing surface agitation would be a deficit. If I were to go out and buy a new filter right now, I would seriously consider a canister filter, even though they cost more.

Michael

I use Aquaclear hang-on-back (HOB) filters on several of my tanks, and have excellent plant growth. I keep the water level high enough so that the outflow of the filter falls smoothly into the tank, with minimal surface agitation.

The worst thing that can happen with an HOB filter is that CO2 in the water is in equilibrium with CO2 in the atmosphere. This is adequate for many, if not most species.

chagovatoloco

Thanks for the reply. I was thinking of hob vs power head with a sponge. No canister filters, never again. I had many tanks with canisters in the past and eventually all leaked...... over a period of 6 years. Rena, fluval, ehime. They are all good but I want simplicity. My hob just works no leaks, no hoses, no priming. So if the worst that can happen is equilibrium with the atmosphere i'm good with that.

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PlantedTank

Well, I think I should say something. I can tolerate opinions different than mine - I just didn't really expect them. I like my HOB too. I guess I'm kind of a perfectionist, and I need to be a little more realistic. So I'll try to keep my water level up and maybe not look so much for perfection.

aquabillpers

Agree with Michael.

totziens

In my personal opinion, if you can keep the surface agitation to the minimum as mentioned by Michael, I don't see any issue using HOB filter in any tank 15 gallon and below. The greatest benefit of HOB filter is it's easy to clean.

What I hate about canister filter is cleaning it. I have to make sure the water pressure in the inlet hose is retained, so that I can hook it back easily with water flowing back to the canister filter. I also have to carry the bulky filter into the bathroom to clean. The benefit is it does not block anything on top of the tank.

Diana K

Planted Tank covers the issue very well in post #2: Think about what happens in the water (fish and plants, giving and taking Co2 and O2) and compare it to the equilibrium with the air.

Remember though, that water that is not moving will not get enriched with either CO2 or oxygen. These gases enter the water at the surface, then diffuse very slowly through the rest of the volume. Only the top inch or less is really in equilibrium with the air. The rest of the water is in some sort of balance based on the fish and plant loads, and varying with their changing needs.

There are other reasons why more surface motion might be desirable in the tank.
Reduces surface scum. The gases won't equalize if they are not in contact with the water. A film of lipids or proteins on the surface seals it off. breaking up that scum and circulating it to get it trapped in the filter and removed is a very important reason to maintain some surface movement.

Daytime: Circulates that thin layer that is enriched with CO2, and mixes it with the CO2 depleted water below.
Nighttime: Circulates that thin layer that is enriched with O2, and mixes it with the O2 depleted water below.
Without water movement only the very top layer will really be gaining or losing gas to the air.

Equalizes the temperature. Especially important if the tank is too hot, this allows the fastest moving molecules to escape, thus cooling the tank.

Keeps the fertilizer and other minerals the plants need circulating, so as the area next to the plant gets depleted more moves in. (This is not so much surface movement, as movement in general)

And in extreme cases... kills duck weed

Overall the optimum balance between CO2 loss (good for fish, bad for plants), O2 gain (good for fish, good for plants (they use oxygen, too), and very important for microorganisms, and other benefits of moving water has usually been found when there is a gentle ripple at the surface. No bubbling, no splashing.
A filter, power head, or even bubbler that lifts the water from the bottom and sheets it gently across the surface does the best job at creating that gentle ripple. I do not much like bubblers, though. To be effective at moving enough water they do splash more at the surface.

2ManyHobbies

I have had two 29 gallon tanks in use for about the past 5+ years with HOB filters, and 2 x 54W CFL. I use a Hagen PH to blow CO2 into the tank. Plants grow like weeds. I keep the water level up and minimize water agitation.