Jump to content

Dissolved Oxygen, the heartbeat of your reef tank


R0B
 Share

Recommended Posts

  • SRC Member

All things in your reef tank react with oxygen. Fish, invertebrates, areobic bacteria all consume oxygen and expel co2. Zooxanthellae create oxygen, and corals use this to help them remove waste. Whilst dissolved organics need oxygen to enable them to break down. As such it's no surprise that Coral reef health (and colouration) has been shown to be heavily linked to the level of dissolved oxygen (DO).

 

Despite it being at the centre of life within your tank, it is one of the least known and certainly least tested parameters in the reef aquaria. So I thought I would take some time to try and shed some light on this interesting parameter and hopefully show why some reefers are now starting to use Dissolved Oxygen (DO) measurements to help them keep their mini ecosystems in peak condition and tune their systems accordingly.

 

First off, what is DO? Well it is the amount of non-bound free floating oxygen within the water column. Dissolved oxygen enters water through the air or as a plant byproduct. From the air, oxygen can slowly diffuse across the water’s surface from the surrounding atmosphere, or be mixed in quickly through aeration/water agitation. From plants it arrives in the form of photosynthesis.

 

What impacts DO? The level of dissolved oxygen is impacted by several primary factors; these being Altitude, Salinity and Temperature.

 

Regarding Altitude, we dont need to worry too much as here in Singapore we are basically at sea level, however, if you moved your reef tank to Denver Colorado due to it's altitude the tank would have 12% less oxygen (note the best place to keep a tank based on this is Jordan). Hmmm does that mean reef tanks in a high HDB is more difficult to run then in landed house hahaha.

 

As for temperature, the level of DO decreases as temperature increases. A reef tank running at 24c will have 6.5% more DO then a tank running at 26c (c.6.42mg/L). Hence, the advice on needing a chiller is starting to have even more validity.

 

Salinity also plays a part, all be it slightly smaller. A tank with 1.026 salinity has 3% less DO then one running at 1.024. Hmmm maybe I should keep my quarantine system at a slightly lower salinity(?)

 

Beyond the primary impacts DO can change based on several other factors.

 

Bacteria.... Clogged substrate or filters will reduce DO as bacteria replicate to cope with the detritus and thus increase their consumption of oxygen as they breakdown the trapped nutrients. Hence abnormal levels of DO can indicate when your filtration or substrate is past its best. Clean substrate helps promote higher levels of DO. So now you have one more reason to clean your sand and move form socks to roller filters!

 

Additional bacteria. Some bacterial blends indicate that there is no risk of over dosing your tank on bacteria. That is generally true, unless your tank is oxygen deficient where additional bacteria can lead to coral discoloration as well as RTN is severe cases as DO is rapidly consumed. So if you add bacteria do make sure you run additonal DO generating methods such as aeration.

 

Over stocking. Fish consume oxygen and produce waste, which then further requires additional bacteria to break it down, consuming even more DO. As such DO levels can be heavily impacted by the amount and size of fish you have in your system, and as such DO can be used to indicate over feeding as well as overstocking. All things being equal if you notice DO dropped when you added in your last fish, and you are close to low DO levels (6-8mg/L) then maybe it's time to stop adding fish (or improve your DO production). Further a sudden DO drop can indicate a fish fatality as decomposition will lower your DO.

 

Poor gas exchange. Gas exchange is key as it is a primary source of DO. Hence a low flow system with little of the correct agitation will result in lower DO. As will aquarium size. Deeper tanks will have lower DO then shallower tanks even if they have the same capacity.. Which is often why low frag systems have better DO then display systems.

 

Medications can impact DO levels. Whilst this is more of a QT thing, it may well be that copper treatments may have to be dialed back if your DO is not sufficiently high enough (possibly rendering the treatment ineffective). So make sure you have high DO by aeration, gas exchange etc.

 

Lack of photosynthesis. If you have no photosynthetic corals then a refugium is a must if you want to manage your DO.

 

So now we have an idea on what impacts DO. What is the result of not managing DO and running a low DO system? Firstly, let's look at the impact on fish .... It is worth noting that some fish can tolerate lower DO then others. Tropical saltwater fish like Angelfish need higher levels of DO to do well (close to 14mg/L) whilst deep sea fish can be more tolerant. As a generalisation, most fish we get in the hobby are from reefs which grow in the top part of the ocean. Hence, the more we move away double digit DO levels the more stressful it is on our fishy friends.

 

Okay (leaving the best until last) what about my Corals? How does the level of DO impact them?? Well, remember I mentioned that corals use oxygen to remove waste. This 'respiration' is stimulated by elevated levels of DO and limited by it when DO declines. Thus, corals living in high DO environments (12mg/L+) exhibit much stronger growth and colouration. Presumably as they are able to expel waste more efficiently they can use their energy for more exciting things like colour and growth! For low DO systems the opposite is true. Scientists have studied the impact of DO on coral floresencnce and helath. They note that colour starts to dissipate significantly at low DO levels (below 6mg/L). They further observed that if DO drops below 4mg/L well that is simply bad news! Whilst corals can tolerate reduced oxygen concentrations below 4mg/L, this is only for a short periods of time and only if it's just below 4mg/L. Exceeding this threshold of time and/or concentration leads to rapid loss of coral tissue (RTN) and mortality. Apparently death of a coral will always occur at DO levels below 2mg/L.

 

But hold on! Doesn't the level of DO go down at night? Yep it does, and this is I feel an interesting point. Scientists have also shown a link between the colouration of corals and the level of low DO at night (aka 'Dark resporation'). They found that corals can physically handle these lower DO levels as they are 'temporary' and the DO levels pick up after dawn. However, they did find that corals suffer pigmentation loss if the dark respiration occurs in low levels of DO. As such keeping DO above 6mg/L especially at night is important if you want your corals to pop! This is an interesting note and promotes the use of skimming for gas exchange (especially at night as I know some people switch off their skimmer) as well as keeping water movements high even at night, as well as the additional benefit of running a reverse light cycle on your refugium for additonal photosynthesis (increasing DO). Anecdotally I have started running two skimmers. One is set to skim whilst the other is primarily set to just aerate. Not only has my ph gone up but also my DO has too! Hint. Don't run two skimmers to skim as there may not be enough organics and you'll simply under skim on both skimmers increasing your nitrates and causing a micro bubble snow storm in your tank.

 

Another impact is that corals grown in higher DO levels (typically found in frag systems) on the whole have higher health (from a DO perspective) and better colouration. The interesting thing is that even if your nitrate and phospate are low, when you transplant that coral in a DO hypoxia tank it will stress out which could lead to discolouration. Another reason to have a good DO producing system.

 

There is much I haven't gotten into, simply because I dont understand it! Hahaha. Such as oxygen super saturation and disequilbriums. But I do hope that this has shed some light on what is an interesting yet under tested parameter. You may not have an DO meter. But simply thinking about improved sources of DO in your tank will be a good start.

 

Anyways hope this helps. Happy reefing.

 

Sent from Singapore Reef Club mobile app

 

 

  • Like 2
  • Thanks 1
Link to comment
Share on other sites

  • SRC Member
Thanks for the sharing. Very interesting. One quick question, does running an air stone in the sump increase DO? If the additional skimmer option works, I think the air stone option works too.

Sent from Singapore Reef Club mobile app


Yes it will (but it could be marginal depending on the total water volume of the tank). As oxygen permeates the water through the surface what you are actually doing with an air stone is agitating the surface and creating more surface area via the bubbles, you are not 'injecting' air per se. A skimmer does this but the surface area it creates is an order of magnitude bigger then an air stone. That said any agitation is better then none!

Sent from Singapore Reef Club mobile app

Link to comment
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

 Share



×
×
  • Create New...