D = fluid depth

Lx = total internal length of tank

Wx = total internal width of tank

Lt = length of triangular area below stairs (to fluid depth)

Ws = width of stairs

On that basis,

total tank volume is obviously:

Lx * Wx * D

and volume of area lost due to the stairs is:

1/2 x (Lx * D) * Ws

The only tricky bit is to calculate Lx

and you could do that with either trig:

Tan (a) = Opp/Adj [where a is the stair angle]

Opp would equal the fluid depth D, and Adj would = Lx

rearranged would give Lx = D/Tan(a)

Or you could use pythagoras

- all depending how easy it is to measure your tank.