The 1-Gallon Aquarium

There are challenges to keeping a very small aquarium. I have had a few fish died prematurely, before I was able to create a stable environment for my fish and plants. The current occupants have lived for over a year in this miniature world.

Equipment:

Tank: 1.5 gallon (empty) hexagonal aquarium by XXX
Filter: Lee's round (goldfish bowl) undergravel filter, driven by a Stellar S-10 air pump (very quiet)
Heater: Penn-Plax 6" 25W heater (overkill, but I couldn't find anything smaller)
Lighting: natural, indirect sunlight through window, direct sunlight about 1 to 2 hours every day
Substrate: fine gravel, with laterite balls inserted

Maintenance routine:

Toping off with tap water every 4 - 5 days, with a drop of dechloraminator (Jungle A.C.E.)
Light gravel vacuum and changing of 1/4 water volume every 4 to 6 weeks
Feeding the fish once a day
No fertilizing since the first year

Plants:

Anubias nana
Java Fern
Cryptocoryne Wendtii
Java Moss

All the plants are low-light requiring, and with my no-fertilizer and window light only setup, they seem to do just fine. They are not growing very fast, requiring very little prunning, but are also healthy, not withering. There is no noticeable algae on the glass.

Fishes:

Currently: 3 Neon Tetras, who have lived for over a year. They seem active and healthy.
Past:

2 Otocinclus with the Neon Tetras, but there were not enough algae to support them.
1 male Betta splenden. I have tried keeping just one Betta a couple of times, but it kept jumping out. I don't think the surface agitation caused by the undergravel filter/air pump suited it.

Notes:

I found it absolutely necessary to have the airpump, not for driving the undergravel filter, but for the side effect of surface agitation. For the first year or so, I tried not having a filter and airpump. Although the water parameters were still good, I observed a very dramatic swing in the pH during the course of the day. As the sunlight hit the plants, photosynthesis drove the pH up to above 9.0, probably through biogenic decalcification. This then dropped back down into the 7's overnight. I decided to add the airpump (driving the undergravel filter), mostly to stir up the water surface to speed up diffusion of atmospheric CO2 into the water. This worked and kept the pH down in the more reasonable range (up to about 8.0). I believe this contributed significantly to a more stable aquarium.
If I were to start over again, I would choose a square aquarium instead of a hexagonal one. Picture taking would be a lot easier. It may be a little harder to find a square one that fits the round undergravel filter though.

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