Dear Mr. Goemans,
I have read a selection of your articles mainly on the subject of DSB and ammonification because I am running a saltwater aquarium and have done so for many years. Over the past 3 years I moved into keeping a DSB rather than a shallow bed and using the traditional canister filters, and I now always have a high nitrate regardless of what I do.
The DSB is around the standard 4 inches of 1mm grain sand. To date I have been concerned where the nitrates have been coming from, even after two weeks of 10 gallon water changes every other day the nitrates would elevate back to 100 - 200mg/l. My stocking level is 2 Convict gobies, about 6 inches long each, 1 Clownfish around 1.5 inch, 1 Sand Sifting star (2 inches), 1 Serpent Star (1 inch body), 2 medium sized hermit crabs, 1 medium toadstool coral and 1 small bit of tree coral. There are also assorted slugs/snails/crabs/worms, mostly hitchhikers in the live rock, which is about 20kg and rubble sized bits.
Currently the UK forums advise me to ditch the sand every 2 years and refresh it, however, I am having concerns over this and believe it may be due to lack of sufficient knowledge. Could you give me guidance on this, and if I should change the sand and or enlarge the grains.
Dr. Charlotte Windsor
Thanks for your email, and it would have been helpful if you mentioned the aquarium size, what brand test kit you're using and what foods are being fed/how much. As for the nitrate problem, I have either heard or see this very problem in many emails, and I doubt very much you have levels approaching 100 - 200 mg/l. The reason I say this is because there are two methods to test nitrate, and have no doubt mentioned this several times in the past to many readers, but its worth mentioning again as some are totally unaware there are two testing methods. And that's because there are two types of test kits, one that measures the nitrate ion while other measures nitrate-nitrogen, which is actually the more important reading of the two as nitrogen is the element more useful to algae.
To make this clear, one needs to realize that nitrate is actually a compound or combination of elements. One molecule of nitrate is composed of one nitrogen atom (N) and three oxygen atoms (O), that's why it's often referred to as NO3 in writings (1 nitrogen and 3 oxygen). Since the atomic weight of nitrogen is 14.01 and the atomic weight of one oxygen atom is 16, the weight of one nitrate 'molecule/ion' equals 62.01, which is 14.01 + 16 + 16 + 16.
For discussions sake, lets take a brand test kit that measures the nitrate ion (the whole molecule of nitrate) and gets a reading of 40 mg/l from aquarium 'A.' Then use a SeaTest brand test kit for example, which measures nitrate-nitrogen and test aquarium 'A' again. This time the result reads 9 mg/l. There now appears to be a disparity between the two readings, however, both test results are correct!
Since we realize the nitrogen aspect is the factor of more importance, its necessary when using a test kit that results in a 'molecule' result to take that reading and multiply it by .2259 to get its nitrate-nitrogen resultant or 40 x .2259 = 9.0 mg/l. If for some reason you tested with a kit that had its result expressed as nitrate-nitrogen and wanted to know what it would be if shown on a test kit that had a molecule result, you simply multiply the result by 4.4 or 9 x 4.4 = 40 mg/l.
And since this confusion still exits, I tend to believe you may not be aware of this aspect and if you have a 100 mg/l reading using a molecule reading kit, your actual nitrate reading of importance may only be about 22 mg/l, which might be more inline with your current setup.
As to replacing the sand, maybe if I were selling sand I would recommend it. And as to water a change, that certainly is not the correct way to alleviate a true high nitrate level, as water changes only provide short-term relief as you have seen. One should keep in mind that nitrate accumulates inside live rock and in the depths of the sandbed, and when bulk water levels drop due to a water change, that nitrate quickly flows out of the rock and sand into the bulk water, usually within a couple of days and high nitrate levels return quite quickly. And furthermore, the sand-shifting Star also helps the nitrate in the bed gain access to the bulk water!
To actually reduce nitrate accumulation properly, one needs to get at the root causes, which is usually overfeeding easily fed foods such as brine shrimp and various flake foods. Sometimes much of it goes to waste. Controlling/proper feeding, and vacuuming the sandbed at least monthly are probably the two most important husbandry methods to help keep nitrate levels in check.
I presume from your mail that your aquarium is fairly small, e.g., less than 50 gallons. If it were my aquarium, would vacuum the sand, and while doing so, remove at least 5 cm of its depth, e.g., one half the sand area one month, than the remaining the following month. If necessary, correct the foods being fed.
There are also good nitrate reducing equipment such as the Aqua Medic Nitratereductor that may be worth looking into if needed, as it only needs its Deniballs replaced once yearly to insure proper operation.
Again, if I sold sand, I might say replace it, but think reducing its depth and keeping it clean by vacuuming monthly and reviewing the foods fed will go a long way in helping to reduce this compound.
Hope this helps.
You're quite right, I did miss mentioning the size of the aquarium, as it's a 40 gallon system with a 12 gallon sump. The test kits are the Salifert Profi Test and also API dip test 5 in 1, which goes up to 100 mg/l. (not 200 my error)
I feed 1 cube of frozen Brine Shrimp or Mysis Shrimp every third day because my stock level is so low. I have never vacuumed the sand, however, following your suggestion I will invest in an aquarium vacuum. Once I remove 5 cm of sand I presume I clean this then return it to the aquarium. I have the Aqua Medic 400 denitrator that I will be connecting once I get hold of a dosing pump as I have totally discarded my canister filter in favour of the Aqua Medic Riff 500 even though I will be discarding the bio balls in the center trickle filter and replacing with small bits of live rock rubble. I also currently use RO/DI water supposedly at 0 TDS
Thanks for the follow-up, and you'll notice the Salifert instructions say that its results are in total nitrate (the entire molecule); therefore, the result needs to be divided by 4.4 to get the nitrate-nitrogen level. Take another reading and then divide as previously explained, and let me know the real result. Probably nowhere near what you thought it to be.
As for the removed 5 cm of sand, do not put it back in the aquarium, as I recommend this 'fine' grain sandbed not to be more than 5 cm deep, even somewhat less is preferable, e.g., 3 - 4 cm in height. Make sure to clean the remaining as good as possible. As odd as it may sound, the deeper bed of fine grain sand probably held down the nitrate level somewhat, but produced a more usable algae nutrient, ammonium. I won't go in to its details, as you are probably already aware of the microbial processes involved. If not, let me know and I'll explain them. But a shallow bed of fine sand is the way to go in my opinion.
The Aqua Medic Riff 500 and Aqua Medic 400 denitrator combined should be an excellent piece of equipment, especially with the bio-balls replaced with small pieces of rock.
This should put things into a better perspective.
The nitrate reading, after dividing by 4.4 equals around 22 mg/l, so you're quite correct, the nitrate is nowhere near where I believed it was!
Also I have been doing a lot of research into DSBs, all of which say 4 inches of sugar size sand is ideal, but in my case it wasn't! In fact, now that my sand height has been lowered and sand cleaned, nitrates are dropping, as they are now 6.0 mg/l.
If the present nitrate is 6.0 mg/l (nitrate-nitrogen), (about 25 straight off the Salifert card), that is 'extremely' GOOD! Be happy, that's quite LOW!