Diagnose slow decline

[syncro]

Over the past 3 months my tank has slowly changed for the worse. I was hoping someone could suggest what to do next.

Symptoms
- amphipod population slowly declined
- chaeto has slowly died off
- cyano and green hair algae on sandbed
- no algae growth on glass anymore (good for maintenance but doesn't seem healthy)
- 2 snails have died
- protein skimmer pulling out less gunk (also more volatile, harder to adjust)

Changes
- added haddoni carpet anemone
- added 2 bengaii cardinals
- changed carbon brands (MarineLand to Eli's caco3 ROX style carbon)
- changed carbon from floating in bag to carbon reactor
- new bucket of salt, same brand
- chemical warfare between haddoni and rbta

The current livestock all appear healthy and eating well, but it feels they are just better coping with an underlying problem that the simpler organisms (pods, chaeto) did not.

Any suggestions? Thanks!

Tank: 40G, 12 months old
Lighting: 250W MH on 8hrs/day, 4 x T5s on 10hrs/day
Temp: 24.3 - 24.6*C
Salinity: 35ppt w/ ATO
Ammmonia: 0, nitrite: 0, nitrate: 0, pH: 8.4
Filtration: Skimmer, 3.5" DSB, live rock, carbon
Livestock: carpet, RBTA, GSP, occelaris clownfish, porcelain crabs, sexy shrimp, snails, hermit crab, dragons breath, chaeto
Maintenance: Weekly 12% water changes with RO/DI water mixed for at least 24 hours
Feeding: Pacifica Plankton and NLS pellets