Formaldehyde is not good choice as it react with unsaturated fatty acids like oleic acid, which might not be able to bind to nile red stain later and perhaps give false results.

I had a same problem with Chlorella vulgaris. Take very small sample like b/w 3~5ul.cell mobility is almost over.

 Mr. Wasif Farooq i tried your suggestion but i facing same problem.

Thank you so much for your valuable suggestion@ Simrat Kaur, Wasif Farooq, Susan Schlimpert.