Algae grows whenever you have water and light, and if you add nutrients it grows even faster. Is the aquarium near a window or do you have a light on the tank?
If you have a light on the tank and only plastic plants in the tank, you will get algae on the glass and ornaments. Live plants will utilise light, reducing algae problems and help keep the water in better condition.
Tanks with live plants need between 6-16hours of light per day but they need at least 8 hours rest. Some good plants to try include: Water Sprite (a floating plant), narrow Vallis, Ambulia, Hygrophilla polysperma and common Amazon swordplants (plant them in the substrate). You do not need any special requirements for these plants but they do better if they get some liquid iron fertiliser (Sera Florena is a good brand) a couple of times per week, but it's not essential.
You can use a clean soap free sponge to wipe the inside of the tank down to remove the algae.
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Fish usually die from diseases caused by a dirty environment or chemical poisoning from poor water quality. How often do you do water changes and do you use dechlorinated tap water for the water change?
How much water do you normally change?
What type of filter do you have and what filter materials are in it?
How often do you clean the filter and how do you go about cleaning it?
On average you should do a 50% water change each week using dechlorinated water that has a similar chemistry and temperature to the tank water. You should also do a complete gravel clean when you do the water change. You can buy a basic gravel cleaner from any petshop. The first picture on the left "01 Standard Siphon", (at the following link) is a basic gravel cleaner. Look for something similar to this. They are not expensive usually around $10-15. You do not need a super deluxe model or anything with fancy gadgets, just a basic model with hose and siphon tube, and a clean bucket to drain the water into.
https://www.thespruce.com/aquarium-siphons-and-gravel-cleaners-1381091
Make sure the aquarium heater is turned off 15minutes before you do the water change, and turn it back on about 10minutes after you have refilled the tank.
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Four small goldfish, a female Betta & a rainbow/ redtail shark should be ok in the tank for a little but they will eventually need a bigger home. However, at this stage I would not bother doing much apart from trying to get the tank to settle down and if the fish are still alive in 12 months time, then look at rehoming them. In the mean time do not buy any more fish.
What do you feed the fish?
At this stage feed the fish once a day, do a 30-50% water change each week, and do a gravel too. Add some live plants or reduce light to help with the algae problem. Keep us updated with how things are going.
If you lose any more fish, take a glass of tank water to the local petshop and get them to test the pH, ammonia, nitrite and nitrate levels. Get them to write the test results down and tell us the results.
You can also take the dead fish to the petshop (in a separate container of water) and see if the shop can spot any signs of disease. Photographing the dead or sick fish and posting a link or picture for us to view can be helpful too. Also try to describing what the fish is/ was doing before it died. eg: fish was gasping at surface and turned really dark in colour.
