Time for the second post on the topic of seaweeds in the aquarium and I will write a part III as well.

4. What conditions are right for seaweeds?

The short answer is: I don’t know yet (and they will vary for different species). I have some clues that add up to a longer answer though. Obviously light is an important factor. Most tanks come with actinic (blueish) light representative of deeper waters. The seaweeds I collect are from shallow water (rock pools) and so are adapted to whiter daylight (as in freshwater planted tanks). I have played around with lighting (see this post) but not in a very systematic way and so am not too sure about what wavelength and intensity is best. Another factor that is important is sufficient wave action. Before I used a Tunze nanostream to create extra flow, I noticed that detritus and algae could settle on seaweeds, see this picture of Serrated wrack Fucus serratus taken from an older post:

High nutrient levels could benefit some microalgae more than they benefit macroalgae (seaweeds) as is the case for plants in fresh water aquariums and could lead to problems. Another factor that must be very important is temperature. I do not have a chiller and my tank is around 25C (higher than room temperature because of the lighting). Seaweeds from rock pools must have not much problems tolerating this temperature for hours on end, but many might have a problem with it for months on end. I therefore will probably buy a chiller next year…  One very interesting parameter that I have not yet seriously considered is CO2. CO2 can be a limiting factor in photosynthesis, that is why many fresh water aquarium keepers add it (in gas or liquid form) to promote plant growth. I found a very interesting article on the role of CO2 in reef aquariums by a very clever chemist named Randy Holmes-Farley. It could be that CO2 is limiting for seaweeds in my aquarium as well and I’ll definitely need to do some pH tests to find out whether this could be the case. The article lists a table with relative rates of photosynthesis at pH 8.7 compared to pH 8.1 for a number of seaweeds. This is 57% for my favorite seaweed Bushy rainbow wrack Cystoseira tamariscifolia. Very interesting stuff indeed and I shall investigate!