  We investigate the structure constants of the center $\mathcal{H}_n$ of the
group algebra $Sp_{n}(q)$ over a finite field. The reflection length on the
group $GL_{2n}(q)$ induces a filtration on the algebras $\mathcal{H}_n$. We
prove that the structure constants of the associated filtered algebra
$\mathcal{S}_n$ are independent of $n$. As a technical tool in the proof, we
determine the growth of the centralizers under the embedding $Sp_m(q)\subset
Sp_{m+l}(q)$ and we show that the index of the centralizer of $g\in Sp_m(q)$ in
the centralizer of $g\in Sp_{m+k}$ is equal to
$q^{2ld}|Sp_{r+l}(q)||Sp_{r}(q)|^{-1}$ for some $d$ and $r$ which are uniquely
determined by the conjugacy class of $g$ in $GL_{2n}(q).$
