I have witnessed close side by side swimming with my clowns. Also, at night with the lights off they often wake me up with water splashing noises. They might be spawning and they are only 4.5 -6" right now.
I have a book called "Hobbyist Guide to Catfish and Loaches"; this book has an article about some clown loaches that have spawned. These clowns were only 6-7". The loach owner did not see them spawn, but one day he found 39 clown loach fry by accident after uprooting a large plant in the tank.
There is another story you can find at
Loachesonline about some 11-13" clown loaches spawning, where around 450 clown loach eggs were found.
IMO, if the water conditions are right it might be possible for breeding in the home aquarium. In both of the cases above the pH was between 6.1 -6.2. Temp was 84-86F (raising the temp is debatable though). Another factor would be darkness.
A known clown loach breeding ground in nature is in the Danau Sentarum National Park in Borneo. This clown loach habitat is only about 100 sq. miles.
Here clown loaches spawn during the rain season. Adult clowns probably travel from the rivers to the flooded estuaries, which have shallow murky waters. The soil of the swamp forest is made up of mostly peat, which can be over 10’ deep in some areas. The thick peat soils would make the pure rainwater very acidic. The pH would be in the range of 4.5-5.5 in these areas.
I have found much of the above info here-
http://www.eii.org/borneo/danausentarum/dsnp/about/about.html
http://www.eii.org/borneo/danausentarum/dsnp/florafauna/florafauna.html
IMO, the size of the female clown loach is proportional to the amount of eggs that she holds. Perhaps a 4" female clown is sexually mature, but she may only hold a few eggs inside her, and the chances are very slim that even one of eggs will ever become a fry. Let's not forget that a mature male clown loach is also needed. I would guess that the male clowns reach adulthood at a latter age than the females.
If I were to try to breed my clown loaches I’d give them an all-u-can eat buffet for a few weeks of snails/bloodworms/fish.
I’d turn my river tank into a breeder tank that would have different water parameters than my main tanks.
I’d drop the pH to atleast 6.0 (from 7.1).
Fill the tank with Malaysian driftwood so that I can drop the pH without the addition of CO2.
I’d do water changes and replace the old water with RO water to lower the hardness.
To simulate rain, I’d set the spray-bars above the water. This would also saturate the water with O2.
IMO, raising the water temp is debatable because in nature rain would make the water temp drop.
I’d probably want about 3 layers of egg crate on the bottom of the tank to protect any eggs from being eaten.
But most importantly, get the camera ready.
I am preparing for this; I have already started breeding snails and stockpiling Malaysian driftwood.