Colin ,you do need more than 15 Amps
You will have
Halide 3x 400w=1200w
T5 8x 54w =432 w
heaters 2x250w=500w
dart 100w (depends to headpressure ,...
Barracude 300w(depends to,.......
That adds up to 2532 w ,which is almost 23 amps ,also keep in mind breakers are designed to shut of @ 80% of the maximum to protect your circuit ,so if you put all that on a breaker & didn't trip there is something wrong with the breaker
You definately need to put the whole Load on 3 15 amp breakers ,to be safe
keep in mind I didn't even add up the skimmer 7 extra nick ,nacks
they do add up.
when it comes to electricity always better safe & sorry
There is many different ways to remedy your situation ,where is the Tank located ,do you have plugs on different breakers around.
Ultimately you can always hire an electrition to wire a small subpannel with multiple sockets enough for your needs ,given the fact you are investing tens of thousands of dollars on that tank ,that would be a cheap insurance &
piece of mind for you
Just JMHO
Regards
Jason