Hey everyone. So we have had our tank for about 1 month now. Complete novice, we have a 2 & a half year old and have been taking her to our local aquarium since she was 6 months old so decided on getting a 56 litre tank.
Good to read about your tank but sorry to read that you've had a couple of fatalities.
The main reason for fish dying is if the water quality is sub-optimum. Fish produce waste (ammonia) which converts to a less toxic form (nitrates) if the filter is properly cycled. (1) To cycle a tank, the safest and most fool-proof method of doing so is to perform a fishless cycle which usually takes 6-8 weeks, after which you can buy fish. [ Details, and to understand the process better, are in: https://www.fishkeeping.co.uk/articles ... hless-cycling-article.htm ] The other method, which sounds like what you've done, is a fish-in cycle, during which there's a large chance of the fish being exposed to toxic ammonia and nitrite (especially if you don't have a test kit to measure ammonia, nitrite and nitrates daily, and do larger and more frequent water changes if required in order to redress this). Unfortunately, exposure to any ammonia or nitrite will have an adverse effect on fish health short- or long-term, and fatalities - the weakest ones will succumb first. That's a strong possibility accounting for the fatalities. (2) Over the course of a week between water changes, nitrates will build up (and toxic ammonia and nitrite become present, especially if the tank is not properly cycled). Ammonia and nitrite are toxic and need to be at 0 at all times, while nitrates should be no more than 20 above whatever your tap water nitrates level is. Action: It's absolutely crucial to have your own liquid-based test kit to measure ammonia, nitrite and nitrates over the course of the week - and especially just before the scheduled weekly water change - to see if these have built up and guide you into whether you need to do more frequent and larger water changes. Strip-based test kits are much less accurate, so definitely invest in a liquid-based one. Read this for more info and advice: https://www.fishkeeping.co.uk/articles ... ammonia-nitrite-spike.htm
Another possibility for the fatalities, although I wouldn't anticipate this occurring so soon, is if you have soft water. The molly and the guppy are hard water fish (and mollies require bigger tanks) and will succumb to an early demise if kept in soft water. The molly requires water of 15-30 German degrees hardness (268-536 ppm/CaCO3) and the guppy 143-536 ppm/CaCO3 (8-30 German degrees hardness). [ Source:https://www.fishkeeping.co.uk/modules/ ... resheets.php?cat=Tropical and www.seriouslyfish.com ] Check your water supplier website to find out what your water hardness is in German degrees hardness and CaCO3. If it turns out that your water is softer than this, then best to get another species if deciding to get more fish in the future - more of the same species of tetra if just getting a couple of fish, or, if getting more, another species of tetra, harlequin or espei rasboras or cherry barbs might be suitable and their colours appeal to your daughter. If you do have hard water, then guppies, ender guppies and platies are fine for your size of tank (but not limited to these options).
It's possible that the water is going green if it's algae due to the tank being situated where the sun gets in.
Aquael filters are good, so I definitely wouldn't see that as being a problem.