Can I connect 2 AAA battery chargers in parallel (to use them as a power supply with higher mA·h)
The optimal solution would be to use rechargeable battery cells that have adequate capacity to run the fish pump for the duration required.  Assuming that you've already got the batteries and chargers and are trying to make this work.

You haven't told us, but I'm also assuming from your post that the battery chargers you have are designed to charge 4 cells in series, rather than individually.  I'm also assuming that this is a backup DC-powered pump and that you have an AC pump that aerates the fishbowl when the power is on.

You need a line-powered DPDT relay with contacts that are rated for the larger of the charger current or discharge current.  Also note that you may want to design some form of low voltage cutoff, as your pump will fully deplete your cells, likely causing cell reversal.

Apologies for the chicken scratch schematic:



When the AC power is on, the relay is energized and the batteries are connected to the chargers as independent series (1P4S x2) strings.  When the power fails, both packs are connected to the pump in parallel (2P4S), increasing your runtime.  Note that there are a few aspects of this approach that will lead to shortened battery life, but hopefully its enough to get you up and running.