How tricky are these blackwater beauties?
I'd like to set-up a blackwater biotope style of tank for chocolate gouramis. I have a 60cm-long tank holding approximately 84 litres. I'm going to use an air-powered sponge filter for low flow and provide cover from floating plants. My water’s pH is approximately 7.0-7.1 and hardness 11-12GH. Should I use RO or rainwater? Any advice on setting up and feeding would be very useful. This is my first biotope style set-up, but I've been keeping fish for six years.
NEALE SAYS: Chocolate gouramis, like a lot of blackwater fish, are prone to infections when kept in generic community tanks. This is often put down to blackwater fish being adapted to environments with a very low pH (pH4-5 being typical for their natural habitat) and since bacteria are less common in such acidic conditions, they don’t have the sort of immune system needed to cope with the opportunistic pathogens present in neutral or alkaline conditions. Wildcaught Sphaerichthys aren’t likely to do well in anything other than soft, acidic conditions. There may be a little more wiggle room with tank-bred specimens, but not a lot, and it’d depend very much on the conditions used to spawn and rear those fish.
Once you have the fish in soft, acidic water they’re not especially difficult to keep in a single-species set up. While shy and slow to feed, they will take tiny live and frozen foods readily enough. Live Daphnia and newly-hatched brine shrimp are favourites, but once settled they will take frozen substitutes as well, including things like bloodworms and spirulina-enriched Artemia. Flake food isn’t reliably taken.
They don’t come from places with lots of aquatic plants, but rainforest streams filled with decaying wood and leaf litter. Bogwood roots and dried leaves will do the trick nicely; whether you choose to use catappa leaves sold in aquarium shops or collect your own leaf litter doesn’t really matter. Alder cones are a good start, with some beech and oak leaves, and before long you’ll create a nice, dark substrate just right for them. It’s worth putting a thin layer of lime-free sand underneath to block out any reflections from the bottom pane of glass. Gentle filtration is ideal, and an air-powered sponge filter is a good choice. Bear in mind, that filter bacteria thrive best in alkaline conditions with lots of oxygen, the precise opposite of what you’re creating here. So, while the bacteria will still be there, and should still be processing ammonia and nitrite, the rate at which they are working at will be low. Keep the tank understocked by a generous margin and use an ammonia or nitrite test kit regularly for the first few months.
Basically, keeping these fish right comes down to water chemistry. Connecting up some gutter downpipes to a water butt is easily done, and a few rainy days can supply hundreds of litres of very soft water. The rainwater will only be as clean as your gutters, so some degree of filtering and treatment may be necessary.
I run the rainwater from the butt through an aquarium net as it goes into the bucket, and then use an old internal canister stuffed with carbon to chemically clean the water a bit for about an hour.