Like any other living organisms, macroalgae require certain environmental conditions to survive, much less grow and reproduce. Particularly in captive environments, where conditions may be sub-optimal, unnatural and unstable, they can fail to grow. Chaetomorpha (or "chaeto") is no exception to this. When especially unhappy, it might fragment into many little pieces that can cause a real mess.
Chaeto is generally regarded as a hardy alga in aquaria. In fact, due to its taste for high-nutrient concentrations, it often grows at a breakneck pace. Still, there are a few things that can stunt its growth or even cause massive die-back. These include:
- Very low nutrient levels.
- Insufficient illumination.
- Poor water flow.
- Incorrect/fluctuating salinity.
- Excessively thick growth of mat.
Many of us use chaeto specifically to remove nutrients. Thus, if the chaeto stops growing as nitrates and phosphates drop to zero, some might simply say "mission accomplished" and brush it off. However, in some cases, it may be that there are nutrient imbalances at play. Think of the concept of "limiting nutrients." Depletion of a limiting nutrient causes the plant to stop growing (and yes, eventually die) even if other vital nutrient levels are still quite high. Many trace elements, if we may treat them as nutrients here, can be limiting. Thus, it is wise to regularly use a comprehensive trace element supplement. For example, iron (which is naturally present in seawater in very small quantities) can get used up quickly, making the chaeto cease to grow even while nitrates are still through the roof.
Weak lighting is a very common cause of poor chaeto health. Of the three major types of macroalgae (green, red and brown), greens such as Chaetomorpha require the most amount of light. Unfortunately, marketing price rather than effectiveness, many "refugium" lights out there are woefully inadequate for greens or frankly any macroalgae. Chaetomorpha will grow poorly if not provided with 12+ hours/day of very intense full-spectrum lighting. Not every species of seaweed prefers the exact same intensity and spectrum; for chaeto, we have seen excellent results using the Kessil H380.
Good water flow allows the algae to efficiently release wastes as well as take in nutrients and carbon dioxide. Its very high surface area (the result of its thin, wiry form) helps with this. However, if water movement through the mass is weak, the plants can choke out, starve and eventually begin to die. So, if water does not appear to be briskly and evenly moving through the mat, it might be necessary to add supplemental flow to the refugium. Air pumps are great for this purpose; unlike water pumps, there is no water intake for the algae to get stuck in.
Salinity is an oftentimes overlooked factor when cultivating macroalgae. By many accounts, rapid shifts in salinity can be detrimental to chaeto. This type of stress can occur following massive water changes where new water is poured in too quickly (especially if it is poured directly into the refugium). If using an automatic top-off system, be sure that the freshwater outlet is placed somewhere just downstream from the algae. Also, maintain a natural salinity level. Some aquarists maintain salinities as low as 1.020 s.g. either in the hope that they will ward off parasites (they don't) or to save money on salt (come on!). A small handful of estuarine-adapted types such as Ulva can handle this; Chaetomorpha, certainly, cannot.
You may be "lucky" and your chaeto grows like mad. You've got powerful lighting, vigorous flow... But then, after doing so well for so long and filling the entire grow space, it crashes. This might have been because of overgrowth. When packed in this way, inner and bottom parts of the algal mat do not receive adequate light or flow, no matter how strong your lights/pumps may be! Chaeto must be harvested regularly—before it begins to crowd itself out. Most aquarists will remove something like 20% of the mass per harvest. This not only keeps the growth rate (i.e. nutrient removal rate) high, but also prevents crowding.
Taking a little care to ensure the health of your chaeto can go a long way. While not bullet-proof, it is extremely hardy and adaptable and will, if provided with adequate growing conditions, never "go bad" on you.