What happened:

One day there where 14 guppies (9f, 5m), then 8, then 2 (males). They lived happily in the tank for 4 weeks, no signs of stress, illness or issues and then they seem to have just vanished, only found one dead male near the inlet. All other fish and shrimp seem ok.

Tank setup:

• 180l (50gal)
• canister filter 400l/h (100gal)
• inline UV sterilizer
• inline co2 with dual valve regulator
• inline elctronic heater
• large air stone

Tank conditions:

• Temperature: 27°C (80°F)
• Amonia: 0ppm
• Chlorine: 0ppm
• Phosphate: 5ppm
• Nitrate: 10ppm
• Nitrite: 0.25ppm
• CO2: 15ppm/green indicator
• PH: 7.2
• GH: 7°dh
• KH: 10°dh

Tankmates:

• 3 Mollies
• 2 Gold Gouramis
• 20 cardinal tetras
• 3 simese algae eaters
• 3 amano shrimp
• 1 Nerite snail
• 3 rummy nose tetras
• 3 small pleckos
• Lots of plants

Previous issues:

• Battled black beard algae outbreak, got it under control with algicides, algae eaters and co2, not been an issue for months
• 9/10 nerite snails slowly died off after introduction(the one surviving been there for 3 months)
• 7/10 amano shrimp died (very quickly within days of introduction, the 3 surviving are ok for 3 months)
• about 2 months ago one gold gourami got slimey gills and a bare spot on the side and died, not sure why, suspect the other gourami.