I need to take images of 29 dpf ZF fry before loading them into a 96 well plate for fluorescence reading. Then later on I process the image and measure the respective fry length.

The fish are dead so autonomous movement is no issue.

Ventral, lateral, dorsal does not matter as long as they are normal to the camera, I am only assessing length.

The solution has to be super quick to implement since I have to use it for hundreds of fry within one "session".

The fish have to be laid out in a way that I can take a picture and then transfer them into the plate in the exact same order so that I know later on when I measure their size in the image which fish/size ended up in which well.

What I did so far is pouring them into a petri-dish with water where I arrange them either in a straight line or in a spiral depending on the number and then take a picture. This has several flaws.

They float around, making it very hard to arrange them in any pattern that I can recognize in the picture and which would tell me to order in which I loaded them.

They float around, leading to some being tilted if not upright making length measurements impossible.

They float around, especially when I start sucking them up one at a time using a transfer pipette which can change their order.

Reducing water volume to a mere film so that they can't float leads to severe issues with surface reflection.

Removing the liquid altogether by pouring them onto filter paper makes nice images but has two huge downsides. Fibers transferred from the filter might fluoresce and if they are not in water I can't pick them up again using a transfer pipette. Using a forceps makes placing them into their wells too finicky.

Is there anything I could add to water to make the fish less buoyant? I am aware that this is mostly a density thing and that this is not likely to be the right path.

Is their a type of material that attracts fry/ makes them adhere to it?

I am thinking electric charge or even some kind of sticky surface that works underwater (with very little force required both for placing and removal).

Does anyone have a fantastic idea for a membrane bottom solution that would allow for a continuous water-flow holding them down?

Happy about any and all ideas.

Thanks

Tim