vanishing cardinal tetras

[waxlyrical]

Can anyone solve a mystery for me?
I have an established 126L aquarium which has been home to four rainbowfish, 2 silver tip tetras, 1 cardinal tetra, and a green goby for the last 4 years. Recently changed sand to gravel and had a big clean but kept filter running with fish throughout and in about 50% of water. Water parameters all OK waited 2 weeks and then added 2 baby red rainbow fish and 5 new cardinal tetras. All Ok. waited another 2 weeks and added 4 more cardinal tetras and 2 swordtails. Complete - or so I thought! 4 of my cardinal tetras have vanished under cover of darkness. No bodies, no signs of illness, water still fine. Who is the main suspect do you think? Only been disappearing since swordtails added but they dont look big enough to eat them. Two of the rainbow fish are a good size but they have lived peacefully with solo cardinal for years. Perplexed!

[Poledra]

Swordtails, hunting as a pair. Watch carefully and you'll see them licking their lips...

[EauRouge]

Swordtails can be buggers, if they are new though then they might not be big enough to eat a cardinal. Can you give us a few more details about the fish, like what exact species are the rainbows and the goby? Are they all fully grown? What are the water readings? It could be that the cardinals are snuffing it and then being eaten, they might not be being attacked.

[waxlyrical]

Ok more info
ph - 7.6
Ammonia - 0 ppm mg/l
no2 - 0ppm mg/l
no3 5 pp mg/l

rainbows are 2 lake tebera which are full grown and about 4 inches long and 2 boesmans which are smaller and about 3 inches. Swords are about 2 inches and cardinals about 1 inch. Dont know what goby is was just called a green goby but had him years he seems to only like algae. Surveyed them all and they all look slightly guilty!

[FermezLaBouche]

I've only ever kept one swordtail (bought as a platy but after a month or so the tail emerged!) and he was a BUGGER. Very aggressive to the platys. I would bet that's it.
By the way, I've never seen a green goby before so just googled them - they are beautiful!