Is your tank only 3 months old? New tank syndrome and or allopathy. A sebae requies intense lighting. It is not advisable to keep different species of anemones in the same system, I know some do but experts advise against it. Did you research these animals before you added them to your tank?
I coppied this off wetwebmedia.
Compatibility:
THE big question with most large anemone stocking is whether they can be maintained with other Stinging-celled life, particularly hard and soft corals... among other Classes and Orders... as well as other Anemone species. The simple answer is yes, this can be done... with certain provisos.
Delbeek (2002) related the keeping of Entacmaea with other species in a large semi-open system of some 350 gallons at the Waikiki Aquarium. ("...including a Merten’s sea anemone Stichodactyla mertensii, several Heteractis crispa, and a large number of E. quadricolor (these had multiplied via fission several times over the years producing over twenty offspring)."). Note the size/volume of this system... Keeping anemones in small systems with other Cnidarians, including other anemone species is NOT advised... there being too much chemical and physical and biochemical-physical negative interaction to warrant the health of all livestock. Starting with initially smaller specimens, taking care to place the "less aggressive" species ahead of more so, distancing all appropriately, and being very diligent re your maintenance (water changes, activated carbon...) allows for more likelihood of all getting along... with the anemone placed last. Even then, the onus is upon you to keenly observe your livestock. Should other stinging-celled life proliferate, grow too close to your BTA/s, they may become very unhappy, try to move, shrink to tiny proportions, or outright die.
More than one cloned BTA can survive quite well in the same system, and this can be not only a pleasurable way of populating your system, but a useful trade-in item to offset your hobby addiction's costs.
Sebae Anemone
The Sebae Anemone, Heteractis crispa, is one of the more difficult anemones to acclimate to the home aquarium, but it can do fine in surroundings where the correct environment is provided. Sebae Anemones require a strong lighting setup and will likely wither and fail with anything less.
