WaterNotes

Fertilizing your Marine Planted Tank

This topic comes up a bit on various message boards and I can tell you right now its got to be the number one headache among freshwater aquarists - getting this nutrient loading thing right and ‘in balance’. If you’ve got a good balance with nutrients so that your higher order plants or macroalgae’s are growing successfully, then you generally can keep pest or microalgae’s out of your tank.

Macronutrients
The macro-nutrients, those elemental necessities that plants need in relatively large amounts to grow are usually the main focus in providing fertilizer to a tank. They are nitrogen, phosphorous and potassium, and are often cited as ‘NPK’ collectively. When you look at any garden store fertilizer you’ll often see these elements referred to in the fertilizer as N - P - K, ie. 10 - 4 - 2. Most aquatic plants require more nitrogen in relation to available phosphorous and potassium. So, when choosing or mixing up fertilizers of your own, its good to pay attention to this need for more N than P and that K is usually the last in line of importance.

As a side note there is another ratio for macro nutrients that can be confusing - the Redfield Ratio. This was developed to describe the relative composition of marine phytoplankton for carbon, nitrogen and phosphorous. The Redfield Ratio looks a lot like NPK - its written as C:N:P. Incidentally, the general formula for marine phytoplankton is 106:16:1, further suggesting that nitrogen is a far more important source to marine plant life than phosphate.

What does this mean to you and your tank? Well, it means that if you’re using macroalgae to suck up phosphate out of the water, they’re limited in their ability to do so mainly by how much nitrogen they have access to in the form of nitrites and nitrates. Nitrogen starved macro’s wont be able to help out much with a phosphate problem as they can’t grow well without it.

It also means, for those culturing phytoalgae, that the use of Miracle Gro, which is really high in phosphates for microalgae’s needs, could mean that you are dumping excess, unconsumed phosphates into your tank when you dose phytoplankton cultures. If you must use Miracle Gro at least concentrate your algae down (usually by letting it sit in the fridge unbubbled for a day or two) via gravity and dose the smallest amount possible into your tank to avoid phosphate driven problems. Or, use something more in line with what the phyto needs - fertilizers based on the Guillard’s or F/2 formula.

Micronutrients
There are quite a few of these and the main feature they share is that they are required by the plant or algae in much smaller doses than the macro’s. Because of this, they are not often the limiting factor in growth in a tank, with the exception of two in the particular case of Caulerpa and Halimeda algaes. Micronutrients include magnesium, iodine, copper, molybdenum, iron, calcium, zinc, etc. With macroalgae there have been reports that certain species are less apt to ‘go sexual’ or release spores if the iron content in the aquarium is monitored and not allowed to fall to zero. The calcified species of algae - Halimeda in particular - need to have good calcium levels in the tank, though this is easy to avoid in a typical reef tank where calcium is also being monitored for the corals.

With seagrass no one micronutrient is considered absolutely essential, though there are some thoughts and rumors for iron and magnesium being important long term. In my own experience with the grass tank I’m finding that the addition of B vitamins - thiamine, nictoinic acid, biotin and B12 - are all important to the growth of stargrass in particular. Vitamins are rarely included in the micronutrient profiling though. I also dose iron for the seagrass and have at least not found this to be detrimental to the tank.

Nitrogen consumption

In keeping with the theory here that nitrogen is the most necessary macronutrient I followed its consumption in my moderately planted seagrass tank over the course of three weeks. With all the measurements given equal weight, the tank works out to consuming about 80 mg of nitrogen per day.

This is quite amazing really, suggesting that a well established seagrass tank, in line with a reef system, could possibly keep up with all the nitrate production from the fish. In fact, if this were to be attempted, the live rock and the seagrass would very likely reach an eventual point where they were directly competing for available N in the tank. This may end up in a not-so desirable situation where excess phosphorous then becomes available to pest algae species as the macro’s and grasses can’t consume excess P in the water if there isnt enough available N.

Phosphate consumption

Phosphate is used far less by the seagrass species I’ve had in the tank and I’ve found over the last week or so that phosphate consumption pales in comparison to nitrate consumption, and works out to about 5 mg per day.

Warning signals

In freshwater aquaria there are lots of warning signals that the plants send out when certain deficiencies of nutrients are being felt by the plants as they grow. I thought I would start a short table of warning signs on this page that can grow as I get more experience with the marine plants:

Red leaf growth
Primarily affecting new growth and new leaf sets, this indicates a lack of proper nitrogen levels. Cause is most likely the lack of available N to produce chlorophyll, causing a shortage of green color in the plant and exposing the red coloration caused by carotenoid anthocyanin (no N in this molecule).

Some good links for sources (I dont use all of these!):

Dry f/2 medium
Paul Sach’s Aquaculture Sources: copepods, rotifers, microalgae, etc
Kent Marine’s Pro-Culture product page
Florida Aqua Farms
Greg Watson - bulk fert chemicals
Seachem’s Fluorish Fertilizer Line