Hi all,

55 gal
3 goldfish - 7" common, 5" comet, 3" fantail
0 ppm ammonia
0 ppm nitrite
20 ppm nitrate
300 ppm GH
? KH
temp 62F
Marineland C220 canister filter
Coralife T5 48" - 108 Watts total
plants - hydra, giant hydra, java fern
substrate - mostly gravel, laterite on tank set up 5 yrs ago, some sand
water additives - NovAqua, liquid vitamin mix, florish excel occasionally
recent medications - treated for flukes with PraziPro last week, involves removing carbon from filter

I recently (a week ago) switched to a canister filter because I wanted to try to maximize the growth of my plants by decreasing the turbulence on the surface. I had previously (months ago) had two T5 light fixtures on my tank, but the plants under this condition grew with very tiny leaves - I think there was too much light for the amount of CO2. Under the current conditions, the leaves grow to normal size.

Up until a couple days ago, the fish seemed fine with the lower aeration in the tank - slow breathing, no gasping. However in the last couple days, their breathing has increased. I noticed this morning that their gills were very dark and they were breathing a lot more heavily. I turned on an airstone, and the situation improved within 15 min. It appears as though the plants and the fish depleted all of the O2 in the night, and photosynthesis wasn't kicking in fast enough when the lights turned on to replenish.

My question is this: if I add more light to the tank (ie the other T5 fixture), will the plants photosynthesize better and put more O2 into the tank? I am assuming that the goldfish are creating plenty of CO2 for the plants and I won't need to dope any in, but goldfish also like lots of oxygen. I would like to find a balance between the fish and the plants. I was thinking that because the partial pressure of O2 is so high, if I can get the plants to saturate with O2, then the fish will be fine. I am also open to running the airstone at night for a few hours to prevent the total depletion of the oxygen if the plants and fish together deplete it too quickly.

What do you guys think? Is light my limiting reagent?

You are over thinking it. Just get some surface agitation going.

It's science! There is a way to figure this out. Plus, I've been trained to over think, I've been in grad school for 5 years now after all.

I'm just a dumb geologist.

...and keep it going. More light would just require the addition of more CO2 and increase the stress your fish are experiencing from lack of O2. Just to reinforce what TexasCichlid said, surface agitation whether by airstone and/or directing the output from your filter so that it creates a good ripple on the surface is what you need continually, 24 hours/day, etc.

^ Yo Texas, I got a question, can you put those glow in the dark or UV rocks under water?

You know those rocks that glow when UV light is shine on them? Can you? PM me.

Many of the UV reactive rocks are relatively unstable in water and they can become looser powders just from exposure to air. Scheelite is relatively common in terms of availability and should be rather durable, however. I am not an expert in that area, however.

Chances are, the additional CO2 retention from minimized surface agitation is still insufficient to run both fixtures. So you will get neither desirable plant growth, nor double the amount of O2 produced during the day.

But let's assume you do see the best case scenario - double growth and double O2.

Light + CO2 = stored sugar. This sugar is used in combination with O2 to fuel growth during the night. More sugar, more growth, and more O2 usage. Inherently erasing much, if not all, of your daytime O2 gain.

In keeping with your goal of retaining as much natural CO2 as possible, I'd go with the airstone for a few hours at night.

But if there's surface agitation, then the tank is in equilibrium with the atmosphere and the CO2 will be lost. There is no point in adding CO2 in this case. I swear I saw a forum somewhere where everyone was practicing limited surface agitation to keep more CO2 in the water.

DarkCobra, I agree, I think I'm going to experiment with an airstone at night and see how it goes.

Don't people talk about this "champagne bubbling" when their plants are growing so well that the O2 is saturated and their plants start bubbling out O2 instead of it getting dissolved? You wouldn't get this with surface agitation.

Yep, "pearling". More accurately, it occurs when the water in direct contact with leaves is saturated with O2. Depending on flow, this is not necessarily representative of the overall O2 level of the tank, may not disappear with surface agitation, etc.

And it's sometimes confused with "streaming", which occurs when a plant has a small cut or nick, and O2 is simply taking the easy way out instead of dissolving; as evidenced by a stream of bubbles from a single location.

So while O2 bubbles are a good sign, don't try to use them as anything but a coarse indicator of how fast plants are actually growing.

I have an extensive amount of surface agitation and I get a constant stream of O2 bubbles from my plants.

I guess I'm confused because it seems as though my fish were being CO2 poisoned, which is something that happens with people who physically add CO2 to their tanks. If I'm getting high enough levels to harm my fish, I should be able to harness it for more plant growth. Goldfish are dirty, I think it makes sense that you could get this much CO2 out of them, right? I guess the airstone approach could be used to more slowly raise the level of CO2 retained in the tank...?

I could also stagger the lights, so that the tank only gets the power of both lights for a few hours per day. In which case, I would need a bigger power strip for all those timers

I can assure you that in a non-CO2 injected tank, your fish are nowhere near CO2 poisoned. You simply do not have enough surface agitation, hence not enough O2 in the column, hence your fish are gasping for air.

surface agiation is good for multiple reason. as cobra stated earlier. ur getting diminished returns by not having it.. it over time lends to a healthier tank

it does not waste all ur co2. just some, and for good reason

increase ur flow u'll also see a lot less oxygen bubbles meaning they are being dissolved.. reduced flow allows more oxygen to form on plants and not be dissolved.. plants act like a water brake they slow flow down around them this also allows the water around them to be easily saturated by oxygen..
so pearling is irrelevant if ur fish are still stressed

There is nothing special about artificially injecting CO2 into a tank. I can assure you that nature doesn't inject CO2 to get plants to grow, there is just a better balance between inputs and outputs than most aquarists attain. Besides, if you have surface agitation, you are actively degassing the CO2 out of your tank because there is only 0.04% CO2 in the atmosphere.