disposable CO2 gas cylinder
Clarke 600 g disposable CO2 cylinder

I ran out of CO2 gas today for the Fireplace Aquarium and swapped out the old CO2 supply cylinder for a new one.  I get 600 g CO2 at a time in disposable cylinders used for MIG welding, and sure enough, the empty cylinder weighed 1200 g and the new cylinder weighed 1800 g.

End of tank dump

I few weeks ago, I noticed the CO2 bubble rate had increased substantially due to increased flow rate through the regulator on the CO2 tank.  Regulators are the first step in controlling aquarium CO2 and regulators work best when the pressure inside the CO2 tank is constant.  Most of the time is this easy because inside the cylinder the CO2 exists in both a liquid and gas phase in equilibrium, and so long as there is any liquid CO2 at all, the equilibrium maintains a constant pressure in the tank of around 860 psi.  When the CO2 is running out, however, there comes a point when there is no liquid remaining in the tank at which point the pressure of the gas phase CO2 gradually drops as it is exhausted.  Single stage* regulators “notice” the pressure in the supply cylinder is dropping and attempt to compensate by opening more fully, however they inevitably over compensate resulting in increased gas flow even though the source cylinder is at lower pressure.  In some cases this failure of regulation can happen quickly and dramatically and can “dump” all the remaining CO2 gas into the aquarium, poisoning the animal residents (the plants won’t be bothered however).

With my set-up the regulation failure is not so severe, and can be easily mitigated by awareness a problem might be coming and by adjusting the flow through the regulator down a little on a daily basis until the cylinder runs out gas completely.  I get more than three months of constant pressure CO2 and then a couple weeks of manageable instabiliy at the end.  I’ve set a 3 month reminder for myself to remind me when to start carefully monitoring the end of this new cylinder.

*Dual stage regulator alternative

A dual stage regulator is designed to maintain constant flow even if the source pressure changes and can avoid the end of tank dump that can happen with single stage regulators.  Dual stage regulators are much more expensive than single stage, can be easily and erroneously confused with “dual guage” regulators, and don’t seem to be designed for the disposable CO2 cylinders I like to use.

Today I shifted the location of the CO2 diffuser from the left wall of the tank to behind the moutain and underneath the powerhead.  The atomised CO2 gets sucked up directly into the flow and actively pushed around the tank.  Previously I had the diffuser on the opposite side of the tank hidden under the plants and whilst it seems sensible to put the CO2 where the plants are, because the flow pattern is circular, that puts the plants “upstream” of the diffuser last in the queue – the water has to do a full circuit to get back to them and by that time all the microbubbles have already reached the surface.  Those plants can still benefit from the dissolved CO2 of course, but they won’t get any microbubbles trapped under their leaves.  The diffuser is also now hidden behind the mountain, which is good, and is shaded by the powerhead to reduce algae growth on the diffuser, also good.

While I was moving things around and doing a cleanout I also took the opportunity to clean the diffuser by removing it from the tank while still active, layering on a few drops of liquid carbon, and letting that go for 10 minutes or so.  Back in the tank this increased the CO2 flow rate from 120 bubbles per minute to 130 bubbles per minute, so that worked well.

The fish seemed pretty happy about the whole thing but it will take a few weeks to get an opinion from the plants and algae.

Notes:  Cute video of a 5-banded barb swimming through the tunnel under the mountain.  The plant growing on the mountain is Bucephalandra caterina.  The carpeting plant at the base of the mountain is Marsilea hirsuta.

Managing CO2 flow rate satisfactorily is particularly difficult for smaller aquariums; it’s very easy to have the CO2 come blasting out, but a nice steady well-controlled bubbling takes some work.  The usual combination of regulator and needle valve can work, and adding in a secondary flow restrictor between the two can be a big help.

Mott porous metal flow restrictors

Mott porous metal flow restrictor
Mott porous metal flow restrictor

Plastic flow restrictors that work “well enough” can be had for as little as £5, but I’ve always been enamoured of the porous metal flow restrictors from Mott corporation.  A metal disc has hundreds of microchannels fabricated into it such that the gas has to squeeze through to the other side.  By controlling the size and number of the channels and the shape of the disc, any desired flow rate can be achieved for a defined gas supplied at a defined pressure.  I managed to score the pictured one from Ebay.

Fireplace Aquarium Mott flow restrictor

Mott calibrates the flow rate against nitrogen gas with an input pressure of 30 PSI which is a typical regulated gas output working pressure.  The one I got is calibrated to 10 SCCM (standard cubic centimetres per minute) which is to say, 10 ml.  CO2 is less viscous than nitrogen, so this restrictor outputs a flow of 12 ml / minute CO2.  I find that for the 8 hours per day the CO2 is flowing through the aquarium (controlled by a solenoid on a timer) I need a flow rate of 6 ml / minute CO2, or maybe slightly less, so this is the perfect flow restrictor for this set-up.  Given an input flow rate of 12 ml / minute CO2 the needle valve has no trouble at all comfortably getting the flow rate down by the remaining 50% needed.  With the new restrictor in place adjusting the needle value smoothly moves the flow rate up and down, with none of the twitchiness exhibited with much higher input flow rates.

Continue reading “Managing CO2 pressure”

About every three weeks or so I have to give the algae a good scrapedown with a credit card blank.  It involves getting essentially my whole arm into the tank with some complicated manoevering to get through the narrow access at the top of the aquarium.  It takes a pretty good hour to get things sorted out.  I’ve been wondering about whether it’s worth getting a magnetic algae scraper to clean the algae without having to get all wet doing it.  Lots of good choices there, with some people advocating for “the flipper“, while the platinum standard (even more expensive than gold) seems to be the ‘Mighty Magnets‘ range.  What puts me off is the fear that they will scratch up either the inside (or the outside!) of the tank, whereas I’m pretty sure the credit card blank is generally problem free.  The other upside to a magnetic system would be getting in behind plants and decorations.  I haven’t been brave enough to pull the trigger on getting one of these yet, but maybe someday…

Lots of great videos about magnetic algae scrapers, but always for glass… bad sign so much silence about acrylic?