A bit more... Were the fish fat when you bought them? Or did they get fat following feeding with dry foods. As mentioned, tiger barbs will gorge on flakes and other dry diets until they are bloated (if over-fed), and may even battle to swim downwards for a while. This should wear off before the next feed! (Make sure you don't over-feed them.)
Female tiger barbs can be very plump when spawning-ripe, so this could also be the reason, if they are old enough and were already plump when you got them. However, maybe yours are too fat for this to be the case? (Are all of your tiger barbs fat, or are some thin? Mature males are thinner - narrower when viewed from above - and slightly redder than females.)
Dried diets can cause digestive problems (blockages) on occasion, more so in some species than others. If you can feed them with live foods, or frozen live foods once or twice a week, this can help greatly with digestion, and is generally a healthy practice.