I have 2 clown loaches in a 260 litre community tank. I bought 3 - 18 months ago, but I lost one to skinny disease

The remaining 2 have always seemed very happy, playing in the bubbles and current, and always jousting to get to the food first.
About 4 months ago they suddenly disappeared, and they only now come out when the lights go off. I have recently tried altering the current in the tank, and also the lighting levels, but to no avail...
Is this just a mood swing, and that's life, or does anyone know a way I can coax them back to daily life?
In terms of hiding places, nothing changed - but I'm going to add more to see if that has any effect. I know that 2 of these guys is not an ideal number, but now I think that rather than add more to the tank, I should let them go to a larger familly - if they don't start showing themselves!
Any help would be appreciated!
Many thanks
Richard