How many µl of tetracycline stock solution and how much water do you use?

Stock solution:
C₁ = (10 mg/ml) × (1000 μg/mg) = 10000 μg/ml
V₁ = ? μl

Diluted solution :
C₂ = 5 μg/ml
V₂ = (25 ml) × (1000 μl/ml) = 25000 μl

C₁ V₁ = C₂ V₂
(10000 μg/ml) × V₁ = (5 μg/ml) × (25000 μl)
V₁ = 5 × 25000 / 10000 μl = 12.5 μl

Volume of tetracycline stock solution used, V₁ = 12.5 μl
Volume of water used = (25000 - 12.5) μl = 24987.5 μl ≈ 25000 μl = 25 ml

(25 mL) x (5 x 10^-6 g/mL) / (10 x 10^-3 g/mL) = 0.0125 mL stock solution

Supposing additive volumes:
(25 mL total) - (0.0125 mL stock solution) = 24.9875 mL water

[Measuring such a small amount of stock solution accurately would be difficult. If this dilution was actually being done in a lab, it would be better to make the desired dilute solution by designing a set of serial dilutions, rather than doing it all at once.]