Losing Fish For Unknown Reasons?

[Paradise3]

Right so background. I have a 30 inch long tank(30x15x12 - LxHxW) and it's stocking one month a go was as follows;
 
6x Threadfin Rainbowfish
7x Sparkling Gourami
1x Croaking Gourami
4x Female Guppies
3x Male Guppies
2x Gold Rams
3x Shadow Catfish
2x Male Platies
1x Rubber Lip Pleco
 
So yeah, a little overstocked but also over-filtered to cope and all small fish.
 
My stocking now is;
 
1x Threadfin Rainbowfish
3x Sparkling Gourami
3x Female Guppies
3x Male Guppies
2x Gold Rams
2x Shadow Catfish
1x Male Platy
1x Rubber Lip Pleco
 
How in the world do I lose 13 fish in one month and only have an explanation for two of them?! My Croaking Gourami suddenly after 4 months in the tank decided to jump and my male Platy that I ost had swollen gills so I treated the tank for parasites and all was fine for 4 days and then the fish started dropping down... One of the Threadfins literally dropped down dead in front of me in the tank... It was happily swimming around and then suddenly it stopped swimming and just died I'm worried, I don't want to lose all my fish but at this rate I will do!
 
If anything, the rams are doing the best right now and I was expecting them to be the first to go with how finicky they are.
 
My water stats(tested on 19th July 2013) are;
 
PH: 7
Ammonia: 0ppm
Nitrite: 0ppm
Nitrate: 5ppm
 
So can anyone explain this? No symptoms on any of them apart from the Platy which had swollen gills and was gasping which I treated for and the rest have had no symptoms at all. My female Gold Ram developed swollen gills so I treated again and she is fine now.

 

[N0body Of The Goat]

You were more than a little overstocked, you had >100cm of adult fish in 88l of water before taking account of substrate, bogwood etc. Even if you were over-filtering, this tank would need frequent big water changes to keep nitrate under control. Squashing so many fish into the tank will increase stress eg. competition for food, increasing the odds of an infection that will rapidly spread through the community.
 
A Rubber Lip Pleco (Chaetostoma spp.) has no place in a tank with German Rams or even Bolivian Rams, many need 20-23C in a tank with strong water current (especially water surface movement), these are specialist conditions for the likes of "hillstream loaches" and fish from the Malebo Pool area of the Congo River.
 
What did you use to treat parasites?
How much has the tank temp risen in the recent UK heatwave and how did you try to combat the heat?
How much water do you normally change per week?
When was the last time you did a big water change >75%?
How are you testing the water, strips or liquid test kits? The nitrate ones need shaking/bashing on surfaces for a good minute or so.

 

[Paradise3]

You were more than a little overstocked, you had >100cm of adult fish in 88l of water before taking account of substrate, bogwood etc. Even if you were over-filtering, this tank would need frequent big water changes to keep nitrate under control. Squashing so many fish into the tank will increase stress eg. competition for food, increasing the odds of an infection that will rapidly spread through the community.
 
A Rubber Lip Pleco (Chaetostoma spp.) has no place in a tank with German Rams or even Bolivian Rams, many need 20-23C in a tank with strong water current (especially water surface movement), these are specialist conditions for the likes of "hillstream loaches" and fish from the Malebo Pool area of the Congo River.
 
What did you use to treat parasites?
How much has the tank temp risen in the recent UK heatwave and how did you try to combat the heat?
How much water do you normally change per week?
When was the last time you did a big water change >75%?
How are you testing the water, strips or liquid test kits? The nitrate ones need shaking/bashing on surfaces for a good minute or so.

 
I knew I was overstocked and frequently did 50% waterchanges, once a week actually. The last 75% waterchange was last week after noticing my female rams gills. The treatment was Melafix. The highest the tank went was 30c from 27c in the heatwave. Liquid test kit(my other halves shop tests for me) and I always shake them well to make sure they are mixed in.
 
Both the Rubber Lip and the Rams are doing well, the Rams have been breeding and I've had them for 40 days no problems. They've bred twice and had the eggs for 2 days last time they bred. The tank has live plants in and has been set-up for over a year.
 
I've had the Shadow Catfish the longest(Since August last year) and the guppies are mainly homebred bar two males. The Rubber Lip is quite happily eating his way through the algae that grows in my tank along with Courgette and other veggies. The fish are fed a mix of Tetra Prima and Bloodworm(Frozen) daily apart from Sunday when they are fasted to clear their systems. I don't understand how everything has been fine up until a month a go when i lost the first fish.