Dear Sir,

It is always a good idea to collect the feces as soon as its voided. However, there are some inevitable losses in the fecal nutrient content on account of leaching which overestimates the digestibility. Also, the leaching of nutrients is a problem as shrimps are usually attracted by chemical cues and are slow feeders. The nibbling feeding habit breaks the feed into pieces before being ingested and thus more prone to leaching. To avoid the overestimation of digestibility we may give a correction factor in the digestibility estimation when we are sure of the time that the animal takes to consume the feed.  

I have attached an article that may be useful.

Regards 

Dear Dr. Jannathulla R.,

When you want to siphone the feaces from tank bottom in order to evaluate digestibility, you should know that feaces might be excreted couple of hours after feeding. In the meantime, right after feding I would suggest you to collect uneaten feed particles by siphoning from the tank bottom. Special care should be taken not to disturb/stress the fish in tank during siphoning as this might influence the postprandial ammonia-N excretion. In this way also you can calculate the number of uneaten pellets and if you know the average weight of a pellet, you may substract this from the weight of feed distributed during feeding. So, you can find the weight of feed (pellets) consumed by fish in the tank. This might help you to get more precise results in digestibility calculations. Remember, leaching of nutrients is important and can effect your results, hence in the siphoning method it is advisable to collect each feaces particles once seen in the tank. The collection of uneaten pellets is therefore important, because you want to be sure that you collect the feaces and not feed particle from the tank bottom.

Hope this helps.