What can be done about smear algae in saltwater?
There are two sorts of brown films in saltwater aquariums:
On the one hand it could be diatoms, which disappear on their own after a short time. In stubborn cases (a lot of silicic acid in the source water) there are two possible remedies: the use of JBL SilicatEx or the application of the synthetic resin MP 600, a strong basic ion exchanger. A mixed bed filter can also remove silicic acid from the source water. Reverse osmosis systems only help to a limited extent.
On the other hand it could be smear algae. Smear algae control needs quite a few words of explanation. Firstly: How can we distinguish between diatoms and smear algae?
A “feeling test” shows simply the difference: Diatoms feel rough, smeary and soapy, whereas smear algae feel soft, smeary and soapy.
The cause of smear algae is mostly poor water quality, which can’t be measured with a nitrate and phosphate test. An ORP measurement can be very useful here. The ORP measurement is a continuous measurement, which informs about the tendency in water quality. Rising ORP values show an improvement, dropping ORP values show a deterioration of the quality.
Example: An aquarium is not yet equipped with a flow pump. An ORP measuring indicates a value of 200 mV. The use of a flow pump creates water movement which whirls up and eliminates “dead zones” behind the aquarium decoration. The ORP value drops for a short time (whirled up mulm) and stabilizes afterwards at a higher level than before (the dead zones have disappeared). An improvement of water quality has been effected (with fewer reductive processes in the water). When additional flow pumps are used, their optimal positions can be determined by means of ORP measurements. The same can be done for the optimization of the filter positioning and with the deposition of silt at the bottom. These both influence the ORP value, but can’t directly be measured with commercial water tests.
If you raise the ORP value by about 150 – 200 mV, the smear algae will most probably disappear. The idea is to try out changes in the aquarium, evaluate the result with the ORP value, keep changing things until you reach the desired result.