Loretoensis Dying Please Help.

[Supercorygirl]

Needing help yet again. Several months ago I purchased 5 C. Loretoensis and 5 C. Similis. I quarantined them for a month as they had already been quarantined at my lfs for 2 weeks, with no meds as my lfs (trusted) had all ready dewormed. All fish eat readily, active, if not to social and only issue was some shortened barbels which are slowly growing back. The tank they were added to is stable and has been running for several years holding various fish without issue as I have used it as a breeding tank several times. Originally supposed to be used to hold just the Loretoensis but the Similis tank developed a leak and they were added in, and have lived together for a month and a half without issue. My problem started 3 weeks ago as I lost 1 female out of the blue, no marks or redness, all fins and barbels intact, not a hint of fungus. I checked my water parameters and all were excellent. So I did a WC and wrote it off. Last night I found another Lorentoesis dead (male), again no marks nothing, and tankmates all healthy as well. The three remaining (2F1M) seem healthy are active, eating and the 2 females show eggs developing through the belly skin. Chatting with a cory breeder friend, I was told they were wild caught as he knows the store and had inquired. Please help with advice and/or suggestions as I'm down to 3 of these beauties and don't want to lose them.


Tank:
L-24" (60.96 cm) W-12.25" (31.12 cm) H-12" (30.48 cm)
15.27 US Gal (12.72 UK Gal) (57.82 Litres)

Tankmates:
5-C. Similis
5-Guppies (dithers)
6+ unknown cory fry believed to be Similis

Fauna/Flora:
Small java moss covered malaysian driftwood
Playsand substrate
Sparsely planted dwarf sag, crypts, swords
2-Aquaclear 20 filters, (media is sponges and ceramic rings)

Maintence:
weekly 50% water changes a few degrees cooler than tank temp of 22C (72F)

Parameters:
Kh-4-5dkh (71-81ppm)
Ph-7-7.2
Ammonia (nh3)-0
Nitrite (no2)-0
Nitrate (no3)-20ppm
No gh as kit is expired

No action taken other than a 50% water change after discovering corpse.