\[ \boxed { {\frac {\rm d}{{\rm d}x}}y \left ( x \right ) =-1/2\,{x}^{2} \left ( ax-2\,\sqrt {a \left ( a{x}^{4}+8\,y \left ( x \right ) \right ) } \right ) =0} \]
Mathematica: cpu = 0.274535 (sec), leaf count = 34 \[ \left \{\left \{y(x)\to \frac {1}{72} a \left (-96 c_1 x^3+144 c_1^2+16 x^6-9 x^4\right )\right \}\right \} \]
Maple: cpu = 0.265 (sec), leaf count = 27 \[ \left \{ {\it \_C1}+{\frac {4\,a{x}^{3}}{3}}-\sqrt {a \left ( a{x}^{4}+ 8\,y \left ( x \right ) \right ) }=0 \right \} \]