It is common for residual levels of nitrite to remain in even well established marine aquaria. That’s okay, and not a cause for concern as long as we’re only talking trace amounts of the nitrite. The acceptable range for the nitrite is generally considered to be anything between 0.000 to 0.100 mg/L or ppm, but when you are consistently seeing levels in excess of 0.05 ppm, it’s advisable to find and correct the source of the residual nitrite.
In your case, a reading of 0.25 ppm nitrite is obviously above the acceptable level, but I’m not entirely certain that your reading is accurate. The color comparison test kits are simply not that precise, and you may want to take a sample of the aquarium water into one of your local fish stores so that they can evaluate the water quality for you using more sophisticated test equipment. That way, you can test the same water sample using your Aquarium Pharmaceuticals nitrate test kit and see if your reading agrees with the pet shop’s more accurate test results. If not, you may want to invest in a new test kit.
Also, if you’re using tap water to prepare the salt water when you make partial water changes, it’s quite possible that your tap water could itself be a source for the nitrite. Municipal water supplies often have chloramines that may affect the ammonia, nitrite, and nitrate readings in the aquarium when the tap water is used for water changes or to top off the tank when compensating for evaporation. So check the nitrite reading for your tap water to eliminate that as a potential source for the residual nitrite.
In the meantime, as long as the fish and invertebrates in your seahorse tank are not showing any harmful effects from the nitrites you have been detecting, I would not freak out about the situation. Your test kit may not be accurately reflecting the amount of nitrite that is present, and residual levels of nitrite are a much greater concern for reefkeepers with live corals than they are in a fish-only tank with a cleanup crew.
Your aquarium maintenance schedule and regimen of partial water changes are exemplary, so I don’t believe that uneaten frozen Mysis is at the root of this problem. In fact, if anything, your maintenance schedule may be a little too rigorous, Amanda. You want to clean or replace the pre-filter or mechanical filter that is straining out the particulate matter from your tank often so that it doesn’t get clogged and reduce the water flow through the filter, of course, but the activated carbon itself does not need to be replaced on a weekly basis. Your activated carbon should last for a good month before its chemisorption ability is significantly impaired and it needs to be replaced with fresh carbon.
The sponge filter will be become biologically active and house a large population of the beneficial nature find bacteria that provide biological filtration so it’s important to rinse and clean it in saltwater rather than fresh water. With a little practice, cleaning the foam filters properly is a snap. Simply immerse them in a bucket of saltwater and gently squeeze out the sponge until it’s clean and releases no more sediment or debris. (I use the saltwater I siphoned out of my aquarium when performing a water change for this, and clean my sponge filters whenever I change water.)
Here is some additional information of the water chemistry for a saltwater require that explains the basic parameters you should strive to maintain, Amanda:
Basic Water Quality Parameters.
Natural Seawater Value = 0.010 mg/L
Acceptable Range = 0.000 to 0.050 mg/L
Optimum Level = 0 at all times
Ammonia is highly toxic to both fish and invertebrates in even small amounts (> 0.05 mg/L or ppm). Causes of ammonia toxicity include: immature biofilter (new tank syndrome), impairment of the biological filtration due to antibiotics and other medications, overfeeding, overstocking and dead specimens that go undetected (Webber, 2004). Ammonia levels can also rise after the addition of new animals, after a water change, or following a heavy feeding. Any ammonia level above 0.05 mg/L is a cause for concern, and the source must be found and corrected immediately. Be sure to maintain a good schedule of water changes.
Natural Seawater Value = 0.010 mg/L
Acceptable Range = 0.000 to 0.100 mg/L
Optimum Level = 0 at all times
Nitrite is slightly less poisonous to fishes than ammonia, but deadly to many invertebrates at very small concentrations. Residual levels of nitrite are common in marine aquariums. Levels of 0.05 or less are of little concern in a fish-only aquarium. If the levels are higher than this, the source should be found and corrected immediately. Even trace amounts of nitrite can wreak havoc among the live corals and delicate invertebrates in a reef tank. High levels of nitrite result from the same causes as ammonia.
Natural Seawater Value = 0.050 mg/L
Acceptable Range = 0.000 to 20 mg/L
Optimum Level = below 10 mg/L in fish-only tanks; 0 mg/L in reef tanks.
Nitrate is the end product of the process of nitrification, formed during the Nitrogen Cycle by the oxidation of nitrite by aerobic bacteria. Nitrate is relatively nontoxic to fishes, but elevated levels (> 20 mg/L) are stressful to seahorses over the long term and promote the growth of nuisance algae. Reef invertebrates can be much more sensitive to nitrate, and concentrations as low as 0.06 mg/L can cause problems for symbiotic stony corals. Any level above 5.0 mg/L in reef aquariums is a reason for concern and should be corrected immediately. The nitrate level is a good indicator of water quality and rising levels of nitrates are an indication of deteriorating water quality. For best results, consider using live rock and/or a live sand bed (preferably situated in your sump) in conjunction with a good protein skimmer to help filter your seahorse setup. The skimmer will remove excess organic compounds before they enter the nitrogen cycle, and live rock and a deep sand bed will provide significant denitrification ability, all of which will help keep your nitrates down. Don’t overstock, don’t overfed, remove leftovers promptly (a good cleanup crew is useful here), grow and harvest macroalgae, practice good aquarium maintenance and maintain a sensible schedule for water changes.
Acceptable Range = 8.0 – 8.4 (typically fluctuates between 7.9 at night and 8.4 during the day)
Optimum Level = ~8.2 and stable.
The pH is a measurement of the alkalinity or acidity of aquarium water. A pH of 7 is considered to be "neutral," neither acid or alkaline, while pH levels above 7 are considered to be alkaline or "base," and pH levels below 7 are considered to be acidic. Marine aquaria need to maintain alkaline conditions at all times, and low pH (< 7.6) is especially detrimental to seahorses because it is conducive to Gas Bubble Disease. Normal daily fluctuations in pH are to be expected in the aquarium, and are generally gradual enough not to be stressful (Webber, 2004). Maintaining a sump or refugium with a reverse photoperiod to the main tank can eliminate these natural pH cycles. Regular partial water changes are the key to maintaining stable pH. Buffers can also help but the hobbyist should beware that excessive use of pH buffers may increase KH values to dangerously high levels.
Acceptable Range = 1.020 -1.026
Optimum Level = 1.0245 for most seahorses.
The specific gravity measures the density of a your aquarium water relative to the density of distilled water, and aquarists use it to estimate the salinity of their aquarium water (Trevor-Jones, Dec. 2002). In effect, it’s one way to measure the saltiness of your tank, since the more salt that is dissolved in the water, the denser it becomes. This can also be done by measuring the total amount of dissolved solids in the water, which is expressed as the salinity in parts per thousand (ppt). Hobbyists must remember that constant evaporation of freshwater from the aquarium causes the salts to become more concentrated, which increases the specific gravity or salinity accordingly. Therefore, it is necessary to top off the tank with freshwater regularly in order to make up for evaporation and maintain the desired specific gravity. Seahorses tolerate a wide range of salinity very well and hyposalinity (specific gravity at 1.011-1.015) is often used to help rid them of ectoparasites.
Best of luck reducing the trace levels of nitrite in your seahorse tank to acceptable levels, Amanda! The problem may be as simple as a test kit that is a bit off or that your tap water itself contains residual amounts of nitrite.