I don't know anything about tiger barbs but, if as Morc says, they do require higher oxygen levels then warmer weather will most likely be increasing the temperature of your tank, which in turn will decrease the level of oxygen in the water.
I dose flourish excel in my tank with no problems but I also run an airline 24/7. Val melts with the use of this product, and because plants increase the oxygen levels in the tank, less plants will mean less oxygen. To get rid of bearded algae you need to dose daily.
Of the fish in your tank the bristlenose will most likely be the best indicator if your oxygen levels are decreasing as they will surface more often and with a major loss of oxygen in the water they will sit at the top with the tips of their noses out.