It is widely reported in the scientific literature that fish eggs are readily colonised by bacteria and within a couple of days after spawning eggs may be heavily overgrown. Bacterial colonisation may have adverse effects on eggs, and on the developing embryo. The use of antibiotics has often been the only recommended solution, but this has given rise both to risks for the environment and for the users themselves, and to the rapid appearance of resistance bacteria.
Fortunately there are other solutions for the aquarist and egg tumblers are used frequently by breeders of mouth-brooding fish (I think these are cichlids) to help prevent fungus. The gentle rolling action caused by these tumblers can simulate the normal incubation method of these fish. One of the major benefits of using this method is that it allows egg incubation in same tank as fish are breeding in to provide identical water conditions during incubation period.
Unfortunately rainbowfish eggs are just too bloody adhesive (sticky) to allow the exact same setup to be used to that you can find demonstrated with devices found on the net. What I have outlined below is a tried and true method that Ron Bowman has developed and passed on to several generations of fish keepers.
What you will need
1. Small tank - just large enough to fit a small heater and glass vessel or jar
2. Glass vessel or jar to contain the fish eggs
3. Stand for the egg container to sit on in tank
4. Sponge filter + pipe with 90 degree outlet, connected to an air supply
5. Strands of acrylic yarn or java moss
6. Breeding pairs of rainbowfish or blue eyes to lay the eggs!
The first step is to gather and sterilize the different components. Set the tank up with the small vessel sitting on the stand. Rest the sponge filter outlet over the glass container. Fill the tank up and glass container with conditioned aged water (possibly residual chlorine in tap water can also be of benefit). Fill the tank to a level of approximately one centimeter or so below the height of the glass container. Slowly turn on air to sponge filter. The glass container will fill and water will over-flow into the tank and circulate around.
The next step is adding eggs to the glass container. Collect the mop or Java Moss bearing eggs from the breeding tank. Gently dry the mop or moss with a clean towel. Carefully extract the eggs from the mop or java moss using tweezers and place onto a single strand of java moss or acrylic yarn. The strand can be then placed into the egg container. Anchoring the adhesive eggs to the strand will prevent them from floating out of the container with the water movement.
The “tumbling” action is adjustable via the air valve connecting the filter and air pump. Adjust the air flow to the filter to provide just sufficient water flow into the egg container to gently move the strands of java moss or yarn about. Don’t use too much water flow as to push the strands out of the container.
The next step is to monitor the eggs for development. It is advisable to remove any non-developing or dead eggs to prevent transmission of disease to other eggs. Within time eggs will develop and fry will hatch out. I’ve noticed more fry will hatching overnight which may be a clever competitor avoidance strategy. The fry will immediately head for the water’s surface where they will flow from the overflowing egg container to the main part of the tank. If pushed for space, it is possibly to feed the fry in the tank and a snail is useful to clean up the uneaten food. Personally I like to remove these fry to another filtered tank where I they can be raised without contaminating other developing eggs. It is advisable to breakdown this setup every few weeks of use and thoroughly clean to remove biofilms and other grime.
There will probably be doubters out there, but I have used this method many times with great success, particularly on fish that have seemingly poor fertility and others that live in environs of very fast water movements such as the Mountain Rainbowfish (Melanotaenia monticola). So if you are collecting eggs but have been frustrated with poor hatching results, this may be a useful method to try.